I think; for myself, at least - doing ‘useless’; or passion/pet projects are a way to ‘keep up your chops’ or the like as a developer, and keep your inspiration for it.

I do SEGA Saturn homebrew, and mostly actually just experiments (I’ve never published any of my Saturn projects so far, though I intend to for one of them), and I only really do it for the obscure interest of learning a kind of fairly obscure and complex processor and co-processor architecture for…the love of it?

I don’t care if I make money off it; I don’t really need to, I don’t care if people see or use it; because that’s not the purpose behind the exercise, and I guess that does seem almost strange but the curiosity and learning is what drives me.

I work a 9-5 as a mobile developer and it’s honestly usually not that challenging or growth-inducing. Not that I’m complaining, I love my job and my company, but in order to stay passionate and ‘into’ programming I find doing these weird passion projects really helps.

Hope that makes sense!

EDIT: I’m also a music producer, and one task I really like to do in a similar way to ‘keep up my chops’ is to re-produce an almost carbon copy of a song I’m interested in particular audio engineering qualities about to, EG, learn about particular techniques being used. I’ll obviously never be able to use or sell them, but I can re-apply the techniques I learned in that experience elsewhere.