Originally Posted by
Alopex
From Vanilla WoW, from my own recollection:
An attack from the front can be dodged, blocked, or parried. Attacks from behind can only be dodged (for NPCs, PCs can't dodge attacks from behind). Additionally, a Parry gives the character performing the parry haste, causing their next attacks to come faster.
Basically, you do more DPS and your tank takes less damage if you aren't standing in front.
This information is also easily outdated
As for PvP, it is for getting attacks in on the back