I think of of it like this:

A problem you experience + Something cool and fun to work on (tech stack/market etc) = Build something

Example.

I live in Norway, and I need to paint my house. A lot of painters out there, some legit, some crap. Prices vary with 50%, so market is not efficient. They need to come and check out the house, then need to follow up, etcetc. Really annoying. So I built a local page for my town, https://maletjenester.no/, where people can paint their house using 1 click, the platform handles the rest. and I am the first customer :)