[GuB-42]

Just look at all the companies that do open source. Google, Microsoft, Facebook, etc... They are all very obviously for profit, if they could sell and become richer, they would, but they still put out open source, on permissive licences no less. It is just that code cannot be leveraged as a competitive advantage, on the contrary, they tend to benefit if people use code they have control over. Why do you think Google insist on people using Chrome even though it is free and 99% open source?

And for hobbyists, most of their code is not worth selling, they are just writing stuff for their own needs and they don't want to spend the effort making a commercial product. So, open source it goes. And if it becomes successful one day, it may still be worth it and you may gain contribution and fame, or maybe not, but you wouldn't have gained anything by keeping that code to yourself either.

Some do it for pure generosity, but it is, I think, a minority. Most either don't care, or it is part of a business plan. Being able to use GPL code can also be a motivation.

[AstralStorm]

Randomly soliciting free help with your decent enough to be adopted project is also a potential benefit. Then there are security benefits as sometimes bugs get found. And you might get free QA of varied quality, the more specialized project, usually the better.

(Though you do get to deal with people trying to strong arm you into improvements or extra work you don't want to do.)