I've submitted series-of-patches to open-source projects that I've worked on before contacting the project.
Partly this is because I wasn't sure if I could even implement the feature. Or, if I technically could, I wasn't sure that I'd be able to see the implementation through to the point it was mergeable (the number of half-started projects I have laying around...)
I didn't want to say "hey, I've got this great idea..." and then disappear into the aether and never deliver. Or, worse, have someone else contact the project with the same idea, and they say "Nah, Karellen is already working on that...", so they move on to something else, and the feature which could have been developed by this other person isn't, so it never happens.
But also, I wanted to give the maintainers a prototype they could look at and play with, rather than vapourware, to get real feedback.
If they say "no, that's not for us", well, I can always put up my own fork if I want to. And I can keep the branch for my local install, which I can continue to rebase or merge into upstream, because the feature is useful to me. Also, I learned a few things by creating the implementation - even if it doesn't go anywhere. The work is no less a waste of time than, e.g. doing the Advent Of Code.
