It is important to understand that the postdoc was not designed to "achieve" anything. It is simply a product of the system's incentives. There is no central planner.
In the US a professor at a research university gets tenure and promotion based on the number of publications and grant dollars he has. As you know, a postdoc or PhD student can write several good papers' a year in which the advisor (professor) shows up a second author. So, someone who can fund 10 students will have 10 times as many publications as someone with 1 student. The more publications you have the easier it is to get funding, so we have a positive feedback loop where the top performers get a continually larger share of the money and talent.
Because of this, a professor would rather hire 2 postdocs at \$40K/year than 1 postdoc at \$80K/year since we know that more money does not translate into more papers. In the sciences and math new PhDs are happy to take the job of a postdoc at very low wages, for several reasons:
- Some really really want that tenure-track job, so are willing to endure the costs.
- Some have few other job prospects
- For many foreign students (India, China) the postdoc salary is much higher than anything they could get in their country, and has the added benefit (for some) of a student visa to the US with the prospect of full citizenship to come. It is much easier to become a US citizen if you have a PhD and are employed in the US.
More than half of US Science and Math PhD students and postdocs are foreigners, so point 3 is a big effect.
There are virtually no postdocs in Law, Medicine or Finance, since someone with a law degree, medical degree, or finance PhD commands a starting salary above \$150K/year in the marketplace, so they are not willing to take a postdoc for such comparatively low wages. In Computer Science the postdoc is still somewhat rare since starting salaries for PhDs in CS are around \$100K in industry. Thus, point 1 also seems important.