Ok so I am not sure if this is concidered common sense, but I just found out and I know out of the people I've told not many have known!
This is useful for farming raidinstance trash, like the pet from zul'gurub, rep / boe epics in ICC or whatever without getting saved to the instance and without having to wait for the instance to reset. I use this to farm the mats for my Sulfuras.
You first invite someone to your raid and clear the trash ones, then you go out of the instance and have someone else invite you, convert to raid and you enter again. Blam, all trash is back! Your friend may then leave group as you clear the instance - and when you're done you have another friend (or the same friend on another character) invite you again. From here all you have to do is repeat!
I used this to clear all the mobs I need in MC every 5-10minutes instead of clearing, waiting 30minutes and going again! Very useful for things like this!