It's the social aspect and the gear progression. When you have done the majority of the story, the small story stuff, especially if you've played the game for a decade, isn't as interesting so you're left with a monthly subscription for... what to run around leveling more toons to see the same content? So there's Raid content. I get better gear so I can beat better endgame bosses. To get better gear I have to raid. With their Random Number Generator, I may not get it this week, so I have to come back next week to try again. Well it's going to be more difficult to progress with people who are just random themselves, so you join a guild and your guild sets a schedule so you know when reliable people will be on.
So if you don't get on at certain times, you wont have a reliable team meaning you wont be able to down bosses so you wont be able to get gear and to see the content.
And you bet your butt I want to fight fel kilrogg, a resurrected Mannoroth, and Achimonde in the next patch. I can't do that unless I'm geared enough to go there. (Plus, BOATS AND SHIPS AND I've been waiting since warcraft 2 to build ships again. and tier 18 looks lame for druids but holy hell does the warlock mythic tier look badass
http://media.mmo-champion.com/images...ythicROUGH.jpg) So I raid to see story content as I'm thoroughly invested in all things Warcraft (working on War Crimes right now, good book, I highly recommend it). Socially as well your online buddies and guildies expect you there too. And if you're closer with them, you're playing with friends. Heck I'm currently losing against a guildie in hearthstone while I'm at work (shhhhh) Then some feel an obligation to be on to help those friends.
Like my boyfriend is my social aspect and he's really invested in it. So I feel obligated to help him out, and I also have fun with it! So win-win!