On some devices, the performance of SQLite database built into Android significantly degrades over time.
I guess it has to do with hardware more than anything else, and none of my devices (new and old) exhibit this.
If the app's UI performance gets really slow, such as opening a folders' message list taking 15 seconds or longer (yes, it really does get that slow), then that's a good reason to run the compression.
Other than that, the app already does nightly maintenance of its database, trimming old data, and asking SQLite to perform incremental garbage collection.
