For example, I care about my meal prep quite a bit and wanted a way to generate a meal plan based on the recipes and foods I had available. While there are a bunch of apps out there that do that for you, I wanted to save the ~$20 and built my own. While the ROI was definitely not there, it was fun to build and I still use it today.
Another example I did was that I formerly was a professor and got asked numerous times for additional practice problems. So I built a web app that would generate questions with random numbers (these were mostly math based questions) so they had an "unlimited" number of practice problems. This turned into a massive project and was incredibly fulfilling.
The number of problems are practically limitless - good luck!