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:
- What format should these presentations be in? (PPT, PDF, etc.)
- What audience should I prepare these presentations for? (introductory, subject experts, translational outreach to industry, etc.)
- How many slides should I prepare for each paper?
- Should I prepare independent presentations for each paper, even closely-related ones, or should I bundle my research into a few thematic presentations that cover multiple papers each?
- Would it be sufficient for me to simply email you a soft copy of my slides, or are you asking me to prepare as if I were giving an actual oral presentation? (i.e. working out the script, patter, timings, etc.)
And finally (perhaps in a follow-up email):
- If this occupies all of my time for [insert number of work-weeks this will take], should I be doing that instead of [insert regular work]? Why?
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.