It varies for me. Sometimes it's just an idea and I want to share it. Maybe I'll never do it, maybe I want to get feedback on it. Some pretty cool stuff came out of just "What-If" like Mongrel2 and Librelist.
Other times I get infected with an idea and have to crank out a prototype to see if it'd work. When I'm done, I still try to throw it up there and see what people think. Usually the process of writing about it actually helps me work out if it's worth it or not as well.
