This is the first add-on I've employed, which I'm puzzled as to why is not a core function of XenForo??
Since XenForo does not automatically clear a user's alerts until they go and purposely hover over them with their mouse, one can read a bunch of posts they were alerted to and all those specific Alerts *will remain* in their Alert Tab. Boo-Hiss!
This add-on fixes that! Once a user has read the posts they were alerted to, all those alerts automatically disappear from the Alert Tab. Simply stated, that's just how things ought to work. It lessens user confusion and saves them the time/effort of manually clearing (now-) unnecessary alerts.