I ran an open source project for years.
A lot of people equate open source with being free (as in cost) but there's also free (as in freedom). It's useful to distinguish these two concepts.
Gratis :- without charge Libre :- having freedom
There's a lot of misconceptions about why people write open source software in the first place.
  • Sometimes it's about transparency (to see the code).
  • Sometimes it's about needing help (to write the code)
  • Sometimes it's about teaching (to show the code)
  • Sometimes it's about distribution (to share the code)
  • Sometimes it's about marketing (to get noticed)
Or of course it can be any mix of the above or something I didn't even think about. Usually the license will give you a hint as to which.
Personally I think transparency is the most valuable. I have no problem paying for software but all other things being equal I'd rather pick open source software simply because I can look at the code and see what it does even if I never want or need to change it.