Dear JIngbo,

You ask why postdoc experience is essential in most cases? 

I think that any answer to your question depends, among other things on the country we live in. In my country (Portugal) for example, it is often the case that people generally apply for a postdoc grant because they did not find yet a job either at a given university or outside the university. This does not happen, for example, in the United States of America where  a postdoc experience is essential for one to get a job of assistant professor.  What about your country?

From your comments it seems to me that to refer to a dichotomy ( i.e., abilities trained during a postdoc period vs. a period for a person to sharpen his/her skills) that is not necessarily the case. As I see it, a postdoc period can be a period in which one trains certain abilities and skills and also sharpens them. Note that when, say, we train  our abilities and skills we are generally sharpening or deepening them. How could be otherwise? As I see it, among the abilities trained or sharpen during a postdoc period are the following.

1. Competence to teach at the undergraduate and/or graduate level. In my country it is often the case that Associate of Full professors have their lectures given by their postdoc students.

2. Ability to learn, namely from the adviser professor.

3. Ability to exchange viewpoints and perspectives with students, peers, and professors. In my country, postdocs have to make presentations from time to time. Of course, this is a an opportunity for one to exchange points of view and perspectives. 

4. Ability to raise, say, "irritating questions" or doubts. If you are acquainted with C. S. Pierce's thinking, you know that for him science progress mainly through the irritation of doubt. As I see them, irritating questions are those questions whose response lead us to see the world whatever from a different perspective.  All creators and innovators, I. Newton, K.Gauss, C. Darwin, J. Piaget, A. Einstein were geniuses and innovators just because they were able to raise "irritating questions".

If postdocs were trained, as much as it is possible, to raise irritating questions, the the postdoc period is welcome and noteworthy.

hope  I have got you question and that this helps.

Thank you, Orlando M Lourenço , for your answers. 

Yes, I agree that it depends on different countries. But most likely if one wants to be a faculty member, postdoc experience is mandatory as I saw many job descriptions.

The abilities 1-3 are also trained, I guess in most cases, during phd study as well. Phd students are obligated to teach, give presentations in conferences, communicate with peers, and self-learning. The 4th one, as a sign of independent thinking, is critical to be a researcher and this should  be trained when people start their phd, in my view.

Your comments are inspiring and helping me identify what skills I need to learn or sharpen. Thank you.

Regards, Jingbo

In my opinion, the experience of postdoc means that you can get the ability to lead your own research group. The ability is not only for the experiment skills, but also for the academic ideas. 

 Hi Dr Huang, I agree with you.