The simple pleasure of sharing their work for people to enjoy. :) Sometimes some aforementioned followers… And sometimes donations - if they enable those. Potential support for their Patreon account, if they have one.
Keep in mind that not every game is the new Undertale. Sometimes you just spend a month on something small and then realize it’s not good enough to ask money for. But somebody might still like it enough to offer up a donation or constructive feedback. Or they see something in it that you didn’t and have some wholesome, free entertainment. Either way, giving a game away for free is better than just deleting it.