The marginal cost of software is basically zero, software that does exactly what you want is extremely valuable. So I'd need to be paid if someone had a feature request that I had no interest in completing, but that's my day job.
I feel like in an ideal world developers are due a fair share of the revenue/profits that their software generates, even if they enjoyed the original works and profit wasn't their intention - However with open source I often see a large organisations making huge profits, with close to zero going back to the authors/maintainers.
I add functionality I want. If someone else wants functionality I'm not interested in, they can either do it themselves, or pay me.
Thanks. What aspect is it of GPL'd code you like? Is it the freedom to use/modify it, the fact the source is available, ideological reasons (and if so what?), or something else?
I've never felt like I've taken advantage of others, and I also don't feel like that when others use my libraries to build products and earn money based on those products. If you're giving away software for free, you can't expect to make any money from it, because it's just that, free.
I'm not a specialist of it, just a book I read years ago.
Basically: you wrote some code and didn't know how to make money of it, otherwise you would have.
If someone reuses your code (according to the license of your choice), there is a chance that you get something out of it. It may be recognition (something to add to your resume) or maybe a job offer or be paid for specific development as a contractor or just to help that firm's dev team make the most of your code.
So releasing your code publicly is an appeal for cooperation. Without doing it, well, nobody makes money out of your code but you don't either.
Classic example: A and B are two persons with no connection. You tell B you will get $1 if he/she accepts that A get $10. Otherwise both get nothing.
Most people prefer having nothing than having the extra $1.
If don't know how to monetize the code you wrote, well... game theory pleads in favor of being in cooperation mode. You may get only a small fraction at first of the profits but by cooperating, you're in the game. That is then up to you to make the relation evolve so that you get your fair share at some point.
The other answers display the variety of what people get out releasing their code. It varies a lot from a person to another.
The difficult part is to stay in an healthy relationship, e.g. where all parties are happy. If you can't stop complaining about being treated unfairly, that won't work. But the firm using your code should know as well its own interest in the relationship, so they should want you happy.
the ability to monetize might come from access to capital or social network. often to monetize you need to have money.
working a job or doing contracting is not going to scale. you can only sell a limited number of hours.
and what you can charge is capped by randomly decided on numbers made up by the CFOs or HR departments and by the crab mentality and aversion to organizing.