I have contributed work I've done in the context of paid employment. Very very occasionally I do so outside of work (maybe I use a tool at home, it doesn't work properly for what I want, I patch it and send a PR upstream).

My motivation to do so is if everyone puts in just a little bit of extra effort to contribute upstream we can get many people making small patches. It's nice for a contributor to see folks care (as opposed to just complaining of issues).