It's mainly there to keep the crank and valve in time when the chain stretches, but another big reason is chain slap. When the chain stretches it can flip around kinda violently(chain slap) and that causes even more stretching and premature wear. Or breakage as I've seen a lot of double rollers do way before their stock single chains with tensioners have. Mine has been in several thousand miles without issues. Anyone recommending no tensioner have reasoning behind saying so??