For me, I made something that I wanted for myself. Originally it was just a script I used myself - I built the web service after, once I realised that this might be useful for others. It's B2C and transactional rather than SaaS, which is against the usual wisdom and but there's upside in that it is a more comfortable, less exigent business to be in.

It's not in my usual domain but still technically interesting - that's a large part of the reason I was motivated to work on it outside of the 9-5. Build it lean so that you don't need to spend too much time on it. Your product will look more focused to boot.