...and why is it so popular? (Substitute DPS with DPS Rogue or DPS Mage as necessary.)
I've never played an MMO, my first encounter with this system was in Dragon Age. I don't remember old party based games (Icewind Dale) having such explicitly fleshed out roles, although it was of course a good idea to have a mix of fighters/mages/clerics/thieves. In recent role-playing games, even class skill sets and talents seems geared towards slotting characters into one of the above roles; it's like the devs are insisting we form parties this way.
One of the reasons I didn't jump into The Old Republic was this. I was hoping for something unique, given how different our conception of Jedi/smugglers from the Star Wars universe is from the above combat roles. (Something like KOTOR would be fine. Not great, but fine.) I don't think there are any tanking types in Star Wars to begin with, except certain kinds of droids. It was disappointing to see the same cooldown based stand-and-cast, swing-and-count mechanics coupled with the tank/DPS/healer system.