My contribution is this:
I play in a lot of bands.
Luckily, most are well established and draw a crowd. Money is being made, and it's only fair that I get part of it.
However, one of my bands is a start-up playing mostly original tunes. Gigs are hard to come by, so to become established (and hopefully work our way into the above category) we do the odd freebie as a platform to get the name out there.
Simple, innit.
(Charity gigs are case-by-case).