The "why" for me is "Why not?" - I wanted to give Lemmy a try, and I enjoy self hosting stuff! I also felt that opening an instance was the best way I could contribute, since my Rust skills are nowhere near good enough to work on Lemmy's backend and frontend was never my cup of tea.
As for how I pay for it, well regardless of whether I ended up running Lemmy or not I already rent two dedicated servers as a way of "keeping up to date" with knowledge that comes in handy for where I work, so I have no intent on dropping them, and certainly not while I work here.