This is something I've always wondered with the cover screen that Gunslinger/Snipers get. When I drop straight into cover screen/crouch on the spot... people like to try and run around 'behind me'. Like it makes some sort of difference. Which from everything I have seen in taking damage, survivability, and dishing damage... it does not.
Even though the 'graphic' is in front of you, it seems to still give you all the benefits of cover even to enemies who are attacking you from behind. However taking hard cover (behind an object) it DOES make a difference if an enemy comes around behind you or not.
As a side note, when you drop to cover screen, they cannot break Line of Sight by simply running to behind you (you turn and shoot them either way).
Be curious to get any sort of official word on "Does cover screen still protect you from rear enemies?"
There are a couple other reasons to go behind, if you play a scoundrel/operative/shadow/assassin, you have moves that only work from behind an enemy. Some people will still run behind just from habit. Other reasons could involve not wanting to be knocked back in a certain direction, but if going behind you makes a better knockback for you vs them, then they just don't know. Cover is a unique function that, if not experienced by the player first hand, can be a mystery.
As for whether it provides bonuses, when you drop into cover on gunslinger/sniper, your Defense stat immediately gets 20% bonus, iirc. That bonus is not direction dependent as far as I know, so should always apply. Ballistic dampeners should be the same as well.