Most recently I used it to check light levels at home in different rooms, to determine where we need to boost or replace LED strips. Sure, there's million Lux meter apps, but Phyphox is better than all of them and demonstrates why these things shouldn't be dedicated apps in the first place. In the past I also made use of EM and vibration frequency displays to troubleshoot hardware.

A complement to that is https://play.google.com/store/apps/details?id=org.intoorbit.... which, once upon a time, helped me track down a source of rage-inducing, late-night high-frequency beeping that was driving us insane - down to specific apartment in a block on the other side of the street. I ended up friends with those neighbors, after teaching them how to disable the alarm clock on their Bluetooth radio when they go away for a weekend.