For me it comes down to a personal itch, which is usually along the lines of "What do you mean, I can't do XYZ???" or "I can't believe this isn't a thing!" or "The current implementations are terrible!" or "I don't believe it when everyone says it's impossible"

I do put in effort to make things useful to other people, and take their ideas to heart to widen the userbase, but ultimately it must be useful to me as well.

I judge its usefulness by the volume of support requests coming in ;-)