When WoW came out in 2004 they didn't really have the equipment for this but now technology is that much faster and they are several billion dollars richer.
Think about it. No more skewed Alliance to Horde ratios, no more waiting for groups, no more grouping with cross-server people who think they can act like assholes because they'll never see you again.
And no more being bored due to nobody else being on if you play late!