There are many achievements, especially in dungeons, that go "defeat boss XXX with no player taking damage from ability YYY". Since you can't retry those achievements on the same day once you killed the boss and got a dungeon ID, you want to be notified immediately when you (or one in your group) fails so you can just go die and try again.
This addon will give you exactly the notifications you need: type /wfd <damage name> and you'll get a chat message as soon as anyone around you is hit by exactly that damage type.
So, for example, before trying Hops on 'Trops, you'd type /wfd "Caltrop Carnage" (note the quotes because Caltrop Carnage contains a blank) to get notified when someone fails.