I zig-zag up a very steep hill to find the gentlest climb it means I'm riding bit farther but when you are on a super steep hill, I'm looking for ways to relieve the steepness.

For instance on a very steep hairpin turn (180degree turns), I go on the outside not on the inside. The difference can be 25+% grade on the inside turn while on the outside it might be down to 15% grade. It's the difference between being able to climb or get stopped!

Another reason to zig zag on a hill climb is sometimes you are looking to launch yourself away from those are riding your wheel on a race. By making these moves, you are hoping to drop the guy behind you. You are hoping that the guy behind you hasn't been paying attention to the subtle grade difference on the road so that you are launching off on a easier grade while the guy behind you has to try to catch you up by going up a steeper grade...It's also mental too when you are trying to shake off the guy behind you.

And if you are on a straight steep hill zig-zagging will reduce the grade because you are traveling more. I was just doing that zig-zagging up cause it was just too steep and I was blowing up.