A Change of Pace
Raiding and playing the same five man instances can get quite boring over time. While several encounters are scripted to have random elements in them, they essentially come down to repeating the same thing over and over. PvP provides a different thing to do – even though some fights essentially come down to the same action again and again.
In order to keep the game interesting a change of pace is needed. PvP provides this. Without it, the game would become stale and bland, with only the PvE side of things to do. This would no doubt draw away a portion of the player base, something that any game wants to avoid.
A Casual Approach
You might only have a few hours a week to play WoW, and that's just fine. PvP provides a great way for those with limited play time to get some nice gear and have fun while doing it. Casual PvP makes for a great way to retain the player base, and thus keep the PvE updates to the game coming. Of course, there is also casual raiding and instance playing too, and that's another fine aspect of the game.
However one thing that PvE casual play can't approach PvP casual play is in the rewards that it earns. It simply is easier and quicker to get good PvP gear casually than it is to get good PvE gear casually. It might take a casual guild four or five months to clear Kara, but only take a month or two to get a full PvP epic set.
This is not to say however that, just like raiders, there are not hard core PvP players. The arenas, especially the higher end competitive arm of it (like the arena tournament), caters to those who want to take a hard core approach to PvP. And that's a great way to spend your time if you're into that. While I personally am more of a hard core raider than a PvPer, I still do enjoy my weekly arena matches.
A Competitive eSport
PvP in WoW is quickly becoming a legitimate eSport. For evidence of this you need to look no further than the arena tournament that is begin sponsored by Blizzard. The winners of the tournament are going to walk away with a hefty sum of money, enough that if they spend it right they'll be able to live on for a year. This is only a positive step for the game, and one that will eventually lead to bigger and better things for both WoW and the larger MMORPG community in general.
For all of the reasons above, I find myself compelled to PvP. I enjoy the game, even if it's not the real game in WoW. It provides me with a change of pace, some easy casual play on my alts, and is the beginning of a competitive eSport. These aspects are, in my opinion, the corner stone to the additional game Blizzard has built on top of WoW, and this is only a good thing.