That, and to be a possible customer for cash sales.
Where all these "Just enjoy the game" posters fail, is their inexperience with end-game Inferno. If you haven't spent at least 50 hours A3 Inferno or above, your opinion means nothing - but you will understand after having done that.
D3 is fun, especially early on, but for power gamers like me with over 225 hours already, the end game sucks. I'm not saying the game wasn't worth my money, but only that it will be lucky to last 10 months... no way people are going to be playing this for 10 years.
After 50 hours of finding 100 dps weapons drops from elites that take forever to kill, along with rings with a socket, and items with "Does 9-15 damage over time"... you start to question why you play. Then you find a legendary or set item with a lvl 60 requirement that is beaten by your lvl 50 gear, and you wonder again why you are playing.
In D2 you could go run normal and maybe get a SoJ (highly unlikely, but possible). Or run Meph, or Pindle, or a ton of other options; again with the possibility of finding great gear. In D3 your goal is to run A4 Inferno over and over and over and over and over... because it is the only place top end gear can drop.
In the end, it's simply kill the exact same mobs in the exact same place for the 0.00000001% of finding something better. No uniques or interesting items either, all just pure RNG equipment, of which I doubt any of you know the names of.