Bad girl for missing (or not paying attention in English) but at least your grammar and sentence structure is good.
100% Mohair ... yes (not a blend) it does stretch but only for the first few times you use it. As for the purpose, it is one of the best materials that I've used and will continue to use it. I've used the Professional's Choice and other neoprene girths for years before going to mohair. I find that the mohair breaths and I prefer natural materials against my horse, although I don't like the cotton girths. The one I use is the Weaver Smart Cinch due to the buckle system. I use the roper style - a little more expensive then others but since they last for at least a few years, it averages out.