[PeterisP]

There are a lot of things that people are willing to do for free, but they wouldn't be willing to do it for a dollar - say, helping someone change his car tire.

There is a psychological difference - if you transfer the mindset from 'helping out' to 'work for hire', then suddenly you have to pay "enough" (which will be a huge amount for many projects) or you'll get less stuff done than you'd get for free.