The reason to divide it up into many categories is to compartmentalize the information. The best example I've heard is take an NSA Cryptography expert. Their clearance level needs to be extremely high. Another extremely high level clearance would be for the engineer responsible for turning the key in a nuclear silo. While both of these people have a need to know extremely secretive things, there is no reason for them to know anything about the other's work. Thus the compartmentalization into 15-20 categories.
