Why do you need to do a postdoc after getting your PhD?
No, it's not like a second PhD, because there's little to no documented requirements, no thesis, no defense, no coursework, etc. 

In the 'old days' (prior to the squeeze on academic funding and exponential increase in PhD candidates), one took a post doc primarily to:


Learn a new skill, technique, or method
Dedicate a few years time conducting research to strengthen one's CV without the added stress of teaching and other tenure-track requirements. 


Ideally, the post doc came out a very desirable candidate armed with a great research program and toolbox. 

Today, a post doc is quickly becoming mandatory for academic-track scientists, as the pool of jobs shrink and the pool of candidates expand. It is not uncommon to find people doing multiple post docs, or people finding themselves in a permanent post doc (or research scientist) position. 

While this is somewhat discouraging, the simple fact that funding is scarce and science can be expensive suggests that the ideal candidate has a proven track record and is likely to secure additional grants. Post docs provide one the time and resources to prove that tangibly.