My supervisor asked me to prepare a presentation for each journal paper I have with him. He asked the same for each other student he has (PhD and postdoc).
This seems to me (and to all other students) strange and nonsense. Why would you ask your PhD students and postdoc to prepare a presentation for each journal paper they have?
I asked my supervisor and he told me because I told you so.
Academics give a lot of talks. Having a presentation ready to go for each paper is very unlikely to be a waste of time. You might not end up using the deck "off-the-shelf," but once you have a bunch of good slides, it's easy to slice-and-dice them as needed. Further, learning to make good slides and give good talks is one of the goals of grad school, so there's a good opportunity here to learn from your advisor.
In fact, one "trick" I've learned is that instead of e-mailing someone a screenshot or screensharing a plot during a meeting, I'll take the time to copy and paste it to a slide and add a few bullets explaining what the result shows and what the takeaway is. Then when I need to form a slide deck, I already have a few key slides ready to go. This almost never turns out to be wasted effort.
I asked my supervisor and he told me because I told you so.
Well, that's unfortunate. I see three explanations here. One, and the most optimistic, is that this was a bit tongue-in-cheek; i.e., you will soon see why we're doing this, so there's no need to explain now. Another option is that your supervisor is just dismissive and rude.
But a third option is that your supervisor thought you were being dismissive and rude of him; i.e., he made this perfectly reasonable request and in response, the term is demanding explanations and second-guessing his judgment. (I assume that the downvotes on your question are because others here have also detected that attitude in your question and are responding accordingly.) If this happens to be the case, I would suggest re-evaluating your relationship with your advisor: you are, after all, there to learn from him (or you don't trust his judgment and should seek another advisor).
Other good answers. Check.
I'd guess that there is the implicit issue that "preparing a presentation" doesn't mean just "being able to stand at the front of a room and explain things on a whiteboard or blackboard"... but means "prepare some slides, in advance".
So, yes, preparing typeset slides is a task. Yes, there are formatting and other issues in addition to the scientific/intellectual issues.
Still, apart from the formatting issue, I do think it is worthwhile thinking about content, big points versus small, and a time constraint, to "sell" your work. Extreme case (as someone mentioned) is the "elevator pitch", where you have a ridiculously small amount of time to "sell" your work to some big-shot who can offer you a job or otherwise affect your career arc.
Job talks are a thing.
And, yes, if you want your advisor to "promote" you when they give talks, it is reasonable for you to give them "promotional material". :)
I'll focus on why it is valuable to do such a thing. A paper in some fields, such as math or theoretical science is very detailed. The detail is essential, but getting lost in the details doesn't necessarily lead to insight. What is the "deep meaning" of this paper? Why is it important?
Having written the paper, it may not be immediately obvious what the answers are to such questions. Reviewing the work and thinking about it again can be valuable if you focus on the important questions. Insight can even lead you to ideas you missed earlier that might lead to future work.
A short presentation "about" the paper needs to avoid the detail to be effective and to focus on the insights that it leads to. For that reason alone it is worth doing, never mind that offending your supervisor is a bad career move.
My previous group never asked students to do this, but we did ask them to prepare an "elevator talk" on their research: something that would explain the essence and importance of it in two minutes.
It's probably so that you (or him) can present at any opportunity.
You might be thinking: "If there's already a paper why bother presenting it? Can't people just read the paper?" Some people are more likely to go to a presentation, more likely to pay attention to a presentation, and more likely to remember you as a person when they watch you present. Self-promotion is very important in science, and doing good talks and presentation is a great way to get your name out there and network.
It's also a way for you to get some practice presenting things in general. Incidentally, when you do your PhD defense and/or apply to jobs, you will be asked to present your research, and a good presentation can go a long way. So your advisor is helping you get started on preparing for this.
As every other answer here as noted, all the other answers here are great. But I'm worried about this pattern of communication between your supervisor and you:
I asked my supervisor and he told me because I told you so.
Let me suggest some follow-up questions for you to ask your supervisor, best by email so that you have a documentary trail:
And finally (perhaps in a follow-up email):
If your supervisor gives you work requests that seem unreasonable, it's not your job to work out what their internal motivation is. It's your job to defend and document why it's an unreasonable work request, and it's their job to convince you that it is reasonable.
While there are many good reasons to prepare a presentation, it is a process which takes time. Every minute of a formal presentation takes between ten to sixty minutes to prepare from scratch, half of which will be oral rehearsal and subsequent changes. A good academic presentation of twenty slides will usually fit a half-hour talk, and will therefore take between five to thirty hours to prepare. Let's say it takes ten.
At my current career stage there are about six half-hour talks that would really sum up and showcase everything I've worked on up to now. Preparing those talks, from scratch, would take me about sixty hours of solid work (at ten hours per talk), which after the usual procrastination factors would occupy two to four weeks or work while I dropped everything else. Therefore, if I were faced with a request like that from my supervisor I would not entertain it, because I can't afford to spend an entire month on a relatively unproductive activity, and I would document that decision to explain it to whatever committee I need to explain it to.
(Having said that, I have actually been presenting my work regularly, so I do have a good cache of slides built up. I'd have to do very little preparing from scratch. If I really were asked to put all my work together into a mega-presentation, I think it actually would take me two or three days to do, so it wouldn't actually be that great an imposition.)
So, over to you. Work out how many slides you are being asked to prepare and estimate how long it would take you. If you run the numbers and find that you can finish it in a day or two, it may well be easiest to just get it done (and move on as soon as possible). But if you find that it would take out a significant chunk of your time from your work then defend your work and tell your supervisor -- all in writing -- that without a very compelling motivation you're not willing to jeopardize your research progress. Be ready to fight if you must.
Previous answers are good, so I won't reiterate. I should also note that, in some disciplines (e.g. computer science), most publications come from conference proceedings and workshops, which, you guessed it, involves giving a talks.
I should also note that you should be submitting to as many conferences as you possibly can. The best way to get eyes on your research is to present it to a captive audience. This helps develop your academic reputation and lends credibility to your preparedness as an academic. Also - you don't want your job talk to be your first academic presentations. I have colleagues who ran into this issue, and it didn't go well. Giving a good talk is a process of trial and error. Do it early and often. I strived for about one presentation per month while in grad school.
As pointed out in the other answers, having a already prepared presentation (slide, or notes for a talk on the board) is everything but a waste of time. If at the moment you prepare it, you will ask yourself "why I am preparing a presentation that I might never give?", here are a first (down to earth) reason.
When you will defend your Ph.D. and want to stay in academia, you will have to apply for post-docs or permanent position. You will be more successful if you give talks and if you are "famous" (not like a super star, just that you already met and people saw you in action). Usually, you do not have so much time to prepare such a talk, for the following reason:
Having an already prepared presentation for each paper you have is beneficial because:
