Supplying a patch, giving feedback for issues, is the least I can do to give back to the people who took the time to write that code.
Because some projects I contribute to are also relevant in my day job I get a deeper insight into that part. Money/financial aspects do not play into that decision but I'd be twisting the truth if I didn't admit the positive impact FOSS had to my career.
Having contributed to FOSS projects in a public way also says something about the person I'm interviewing. If most of my stack is built on FOSS and an applicant comes along with a cool CV but has not one contribution to a single project it's immediately a hard pass from me. I never put applicants through silly technical screening sessions unless they are fresh outta university. But no public commits are a red-flag for senior engineers.