This will be a very I-centric post.. but that was the OP's intent, so here goes:
I write addons whenever I feel the stock UI lack something which I take for granted and there is no other addons providing a solution satisfying my own
personal needs.
I write addons
because I can.
I release addons to the public because I love getting comments about why my addons can't do this and that and how people hate the fonts I use or the features I added or forgot to add
I release addons to the public because I hope the stuff I wrote can benefit someone else in the same boat as me.
I release addons just to see how many favorites and downloads I can get (disappointingly few compared to Garkin's addons (he's the king))
I don't do it to learn Lua... Lua is just another scripting language with some pros/quirks and I don't really see myself using Lua for anything else than writing addons which is a pure hobby.