Testing that survives contact with real devices
An Android app that behaves perfectly on a current Pixel can fail on a two-year-old mid-range device with a different manufacturer skin, a smaller heap and a battery optimiser that does not behave like the reference implementation.
Android app testing is the work of finding those failures before your users do. Doing it well needs two things that are awkward to arrange: enough testers on meaningfully different hardware, and a way of collecting what they find that is not scattered across chat threads.