u do get aggro. if the healer accumulated a million points of aggro, provoke will instantly give u 1000001 points of aggro, and thats extremely useful.

as to how to use it, it depends. if the mob has been on u e whole time, but u accidentally lost a little aggro and it starts going for the healer, it is probably unnecessary to voke (unless healer is dying) - in that case, a simple RoH is usually enough to claim it back.

but if the mob came out of nowhere as an add while you are fighting 3 other mobs, and u didnt notice it until much later, chances r that the healer has already accumulated a large amount of hate, so u need a instant aggro boost with voke, then build with RoH on top of thay to secure hate