Arm warmers and leg warmers are a fast and easy way to adjust your clothing if the weather conditions change. For example I commute by bike, and often in spring and autumn it'll be chilly in the mornings but I know it'll be warm enough to ride home in a shorts and short-sleeved jersey. Then I'll add arm- and legwarmers for the morning ride and stuff them in my pack for the ride home.
They take very little space, can be stuffed in a jersey pocket and are easier to peel off than a jacket or tights. If I know it's going to be chilly all day I prefer wearing tights or a long-sleeved jersey, though, it's a little more comfortable than the extra elastics on the warmers.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett