While you don't have to be behind, to use a Mutilate.

If you can arrange to be behind, you'll land a lot more hits where the mob does not dodge, parry or block.
And you'll eat no ripostes.
Most mobs also have nastier abilities in the frontal cone, which is why tanks turn them from the raid.
Being behind means you don't take these types of attacks either.

With Mutilate, if you cannot get behind, you can still land your blows fine.
But if you can get behind the mob, it is an immense advantage.
That goes for all melee classes, using daggers, maces, claws or polearms.