I guess you could call it a hobby as much as anything. It feels more serious than that to me, based on the amount of time and effort I put into it. In addition to developing os-level projects for Plan 9 itself, I write interactive fiction games, which feels to me like a serious artistic pursuit, but most people would put interactive fiction text adventures in the "hobby" category. My brain doesn't really follow the dichotomy of using whether or not something is profit-making for how "important" it is. I understand that is an attitude that is somewhat a measure of personal privilege to not focus that hard on economic issues.

I use 9front mostly because it's a nice place to live. Most of the stuff I do doesn't run on Plan 9, but I to the development on a 9front system. Compared to FreeBSD (I don't have much Linux experience) the system is simpler and easy to write for. The tools are well thought out and simple, plus it has my favourite text editor. I work on 9front entirely because I use it. Not working on it would be like not repairing your home if it breaks because someone else built it. If you have the ability to help make and improve the projects you live in, I really think it's important that you should. This is basically the raison d'être for 9front's existence.

Some people use it seriously for their home/work computers, and work on it solely for that purpose. Others do it out of interest.

It's a very, very nice OS to experiment on due to the simplicity of everything. The kernel is almost small enough to memorize.