Used to use them years ago when on a long ride. Rolled up on top of your saddlebag until needed. Part of a cyclist's survival gear as it was versatile. Could be used as a poncho if necessary. Used as a cape it could keep your bars and hands dry, and cover your saddlebag while you trundled along like a tent on wheels. Best on a low wind, cool day. You had to know where your controls were without seeing them eg downtube shifters, when you took your thumbs out of the loops to change gear. It's a lot easier with Brifters. Also useful if you have to fix a puncture in the rain to provide a shelter to keep the tube dry. An emergency extra layer if the weather suddenly turns cold. Handy for long rides or tours. Mind you, my last one became brittle from being on my saddlebag unused for years, and fell to bits. I ought to get another one, and maybe get out more.