Just simple throwaway apps for myself. For example, an app to control my soundbar. Or an app that helped me figure out which WIFI hotspot to use for better internet during camping. It's not that deep :)
Yes, I used it to track WIFI signals and download/upload speeds over a period of two days. It stored the results in a log file and I then let ChatGPT inspect the log file. Based on that, I then ordered a WIFI extender. Now I have the perfect WIFI at our (permanent) camping spot. :) Didn't use the app afterward anymore.
Maybe I'm just old and not using my phone to its fullest, but I just went through my apps and I could not think of a single example that would be worth building and maintaining my own apk for.
Personally I can't remember the last time I needed an app and there weren't already several options to choose from.