your synchro's don't need to do any work. It's how people had to shift gears before synrcomesh transmissions came along. Also makes downshifts buttery smooth with practice when you can blip the gas to the right rpms. It's just a pleasing experience to do it all seamlessly and second nature and have a seamless downshift.
Also, the manual trans I used off my TT clip is a bit "crunchy" when changing gears. Double clutching makes is shift perfectly. Once you practice double clutching for a while you don't even think about it, just like learning to use a clutch; you don't have to think about that anymore, right?