Bikers wear leather for abrasion protection in the event of an accident. Its heft also means it doesn't flap at high speed, particularly if tight fitting as rec

Bikers wear leather for abrasion protection in the event of an accident. Its heft also means it doesn't flap at high speed, particularly if tight fitting as recommended (again, in case of an accident). But it doesn't stretch, so it's not easy to wear more layers underneath. For protection against the elements, technical clothing with fancy fibres are a better option - way more waterproof, and can work with a heated vest if it gets very cold. Not as good in a long slide though.