To trick 'em. Dean Smith's teams used this philosophy quite often. They were constantly changing defenses to try and confuse the other team and offset their offensive rhythm. The reasons are the same but the zone has really changed. Today's zone is truly a match-up zone, where players have to get down, stay in a stance and pressure the basketball.
Defenders never guard an area.
Defenders in an area where there are no offensive players are pretty much defending air. Although that may be all that some players can defend, most coaches would prefer their players to at least attempt to defend somebody. Many coaches today are recognizing the advantages of the match-up. You almost have to. Even though reason number 2 still holds true in today's game, almost every team has at least one shooter that has to be guarded on the perimeter.