I think PhD is the first time, mostly, when a person does original new work and in this process they gain the skills to do research. So, I was wondering why do people do postdoc(s) when they already have those research capabilities which they can practice even being a Professor or in Industries. Is there something else left to be learned?
Because the requirements for the majority of professorships are such that very few people can be considered serious applicants right out of their PhD.
Also, the number of PhD holders looking for a professorship far exceeds the number of available positions which creates a "backlog".
The two are obviously correlated since the higher the demand to offer ratio, the pickier the institutions can be.
So even in the case where people developed the necessary skills to conduct professor-level research and teaching during their PhD (which should not be underestimated, that level usually requires more experience than what a typical PhD can give), they usually need to spend some years conducting independent or less closely-mentored research along with students mentoring and teaching as a postdoc until they can secure a position.
Note that the majority of postdoc researchers never get a faculty position.
In the sciences, the simple answer is the production of Ph.D.s far exceeds the number of faculty positions (see here). In the days of yore (1950's - 1970's), academia was expanding and new Ph.D.s could more easily land faculty positions.
I believe the postdoc position was invented as a natural way to take advantage of the oversupply. What professor wouldn't want to have a seasoned academic researcher who has already published on the team, for a low rate? It's true there are more skills newly-minted Ph.D.'s need to develop, as noted in the other answers. But that was the whole point of junior faculty positions.
The more recent advent of postdoc positions in the humanities is discussed in this article.
The answers by Cape Code and Paraquat cover some central reasons someone would pursue a postdoc, but there are some other reasons--as well as reasons a school might rightfully find that post-docs are better prepared to be research professors.
Diversification of research experience. If your only training in independent research is your PhD advisor's research, it can be challenging to strike out in a new area. Most PhDs don't want to be a copycat of their advisor's research: it leads to interpersonal tensions, and you're more likely to get scooped when you're starting. Taking your expertise to a new field in your postdoc can be valuable in:
- Broadening the research techniques you're familiar with (e.g. specific instrumentation, models, or lab measurements)
- Broadening your field of expertise
- Developing your own core research questions, which are not copies or derivatives of your advisor's
- Contributing your specialty to a new field to make an impact with a different perspective
Learning How To Manage Researchers When you're a graduate student, you're in the trenches doing the actual work. When you're a professor, you're managing a group of people who are doing the work. Managing well is very tricky. Are you going to micro-manage them? This might get the work done faster, but then you're preventing your students from developing independence, because you tell them exactly what to do at each step. Also, you might be missing out on important research questions that they are asking and interested in pursuing. Are you going to be very hands-off? They might spin their wheels unproductively, and even quit the program after you've spend the time and money training them. How do you find a balance? Do you think happy and satisfied and curious graduate students do better work? How do you make sure they are happy and satisfied and curious while still getting work done and papers written? Effective group management is a set of skills that you are not typically taught as a graduate student. However postdocs often mentor graduate students and could even manage a small research focus within the group, and this can build these management abilities with guidance from the PI.
Experience Writing Grants. Sometimes graduate students write grants or help a professor write grants, but not terribly often. A postdoc can be a good opportunity to co-write a grant proposal with your PI, to give you experience in grant writing. Being able to fund your research as a professor is integral, and how to write grants effectively is a skill that we don't typically teach graduate students.
Becoming more of a presence in your field. To be a successful professor you need to: get funding, publish papers (i.e.: get papers through the peer review process), maybe find collaborators, be invited to give talks, etc. That all comes a bit easier if people in your field know who you are. There's a lot of work to do to set up your lab and recruit graduate student as a new professor, and you'll also have departmental obligations (like committees to serve on), and teaching requirements. If you can start getting your name out there as a post-doc by going to lots of workshops and conferences, it's a little less pressure on you when you're trying to get all your other ducks lined up for tenure.
The following description is valid at least in Germany as I experienced it in the natural sciences. I am not sure whether it is also the case in other countries.
In Germany, you have to do teaching when you have a professorship at a University. There are a few exceptions. Additionally, you are expected to lead a working group, attend faculty meetings, supervise PhD students, grade Bachelor's and Master's theses, obtain third-party funding and probably some minor things I forgot. It is a secure position and you earn good money but there is not much time left to do research .
As PostDoc (but also as senior researcher with permanent contract) you also have some of these responsibilities - but not all and often you can choose more flexible what to do. The important aspect is that you do have more time available for doing research (compared to being a professor).
Moreover in the past, a "Prof." (and also a "Dr.") had a very high reputation in the society. It changed in the previous two to three decades. You have to deals with students -- particularly in their first year -- (and in recent years their parents!!!), who claiming for good grades and a higher quality of the lectures. The latter is actually a positive aspect. However, in my experience people are often not adapting to the university-kind of learning and expected highschool lessons instead of lectures (but that's another discussion).
Summarizing, a professorship a German universities is better paid and a more secure job than a PostDoc position but it also brings a considerable amounts of responsibilities taking a lot of time, which you loose for your research).
In many countries, you need a good publication record to be considered for a permanent position, and the only way you can get a good publication record is though postdoc work. There are also far more applicants than there are permanent positions (in all of the places I have worked, anyway); a postdoc once told me of a job interview he had in Oxford, where he met two senior professors applying for a junior permanent position; needless to say, they weren't too pleased to see each other. I don't know what it's like in the humanities, but for some reason, many jobs, both permanent and contracted, prefer applicants who have had some degree of mobility, i.e. have done research abroad, and a postdoc is a good way to do this, while building an international network.
To understand the question it is important to understand the role of postdoc in an academic career. How I see it:
- PhD student
A training to be a researcher. May end up as a researcher outside academia. Even some business manager positions require MBA or PhD.
A full time researcher within academia.
- Tenure Track (Associate etc.)
A gradual change from a full time researcher to a full time manager.
The managers of the university.
PostDoc is a way to keep the researchers within academia. They conduct the best research because they are no longer students, but they have none of the managerial responsibilities of a professor. As always, things may vary between every parameter you can name (time, place, field, attitude...).