The pros are almost guaranteed to have a few races each season in the rain with temperatures down near freezing. Most pros just try to survive those days but Castelli made this pant so you can thrive on days like that.
When you wear it together with the Tempesta Jacket, you keep your entire core warm and dry down to your knees.
There is a minimal aero trade-off, so they have made the pant so it can be taken off in less than 10 seconds while on the bike. Castelli weren't planning on making this pant until the Cannondale Pro Team riders said they needed it for cold-weather racing. They get it.