it doesnt *have* to be used at low health, but you get the best benefit from it when youre low on health.
which, depending on the fight, may or may not come into play (and if it does, youre probably going to be dead anyway).
given the choices for pve on the horde side, in terms of a dps warrior, orcs are hands down the best. more expertise with axes and a passive racial that gives you a nice AP boost at the cost of an MS debuff on yourself for its duration (which is way better than berserking, since as a dps warrior if you get hit in a raid youre generally dead regardless, so having -50% heals is no biggie).