Why? Well, mostly it comes down to features. Both the Blizzard calendar and GroupCalendar support the basics -- raid lockouts, holidays, player-created events with attendance lists. But like most things Blizzard, their calendar is fairly simple, and more or less ends there. GroupCalendar goes a bit farther.
- Durations and level ranges can be specified
- Finer-grained control over invitations; for instance, you can specify a minimum and maximum quota for each class, with a standby list if you're full on (say) Hunters
- Automatic invites and queuing when it's time for the raid
- Edit: People can be allowed to sign up to an event even if they weren't explicitly invited (this is not the case on Blizzard's calendar, and theirs only allows you to invite up to 100 people)
Don't think this is an advertisement for GroupCalendar, though. If you don't need the extra features, there's no reason to install it. Blizzard's calendar is more lightweight, already built-in, and much more polished (as you'd expect). There are no synch issues, and no bugs (that I've seen). In fact, the only thing missing from the built-in calendar that I really want is for it to show the daily dungeons and battleground. But if you do want the extra features that GroupCalendar offers, the developers have shown no sign of slowing down; you can get it wherever fine add-ons are hosted.