yes it is crucial, for one, it makes it a hell of a lot easier to get bearings out, and it lets you tighten the axle the full amount while still letting the bearings spin freely.
yes as your bearings will last about 1/4 of their life if that if you dont alleviate the sideways pressure that no spacer creates and the idea is to have less friction which will make you go faster
you dont need them but its alot better with them. You can tighten your wheels as much as you want and your bearing will last longer. But its your choice you dont need to use them