Left hand drills are not necessary, but they can be faster.
When drilling out a busted fastener, with a standard twist drill one might need to step in 1/64 increments until they can pick out the spiral out of the threads of what's left of a fastener. A left hand drill has a tendency to walk what's left of the fastener out a bit quicker since its already biting in a direction that is to back out the fastener.