On other rulers, it may only be for the glare, but on the Olfa Frosted Advantage rulers, it's for both glare and to prevent slipping. And it does work; at least it always has for me.
If that's the ruler you're using and it's slipping "all over the place", then perhaps you should try changing the way you are holding it when you cut your fabric. Try putting more pressure on your non-cutting hand and 'walking' that hand up the ruler to follow the blade as you cut. Both those methods will eliminate virtually all slippage with any ruler.