The right-handed (RH) thread vs the left-handed (LH) thread in screws
Differences:

Righty Tighty, Lefty loosey doesn't work with left-handed screws.
Right-handed screws move away from you if you rotate them in a clockwise direction, whereas left-handed screws move towards you.

When to Use Left-Handed Screws
When fretting induced precession would cause the right-handed nut/screw to loosen, a left handed nut/screw should be used.