Why? Simple: sometimes your updates corrupts tables of the Places.SQLite that the about:support's "Verify Integrity can do nothing about! So either you improve the tools for Places.SQLite recovery/repair, OR have Firefox Sync also synchronize Frecency!
And just recently it was stupid as hell how the update corrupted it: made it so the top two suggestions in the urlbar could not be removed AT ALL. Sure, "just remove it from history sidebar", yeah, that works, but whatever goes up after that becomes unremoveable next. Had to flush Places.SQLite COMPLETELY to fix the issue, and ALSO lose all frecency in the process.
