In the balancing game that is drum mixing, successfully isolating the different elements of a drum-kit is crucial. It allows for more focused processing (eq, compression, saturation), tighter sounding kits (less cymbal wash) and easier mixing. For example, you don't have to worry about how a 3-5db boost at 4-6khz in a snare (to get a snappier sound) will affect the overall tone and harshness of your hi-hat. This saves you time from trying to find the best compromise for that harsh cymbal sound (de-esser, dynamic eq etc.) and also improves your overall drum sound.