Future updates increase level caps, unlock more dungeons, items, upgrades, etc.
I'm not a big MMO player, but from what I know, the real stuff starts there. Hardcore version of the dungeons, 40 people raids, stuffing, levelling new characters, HL PVP, etc.
Updates are there to relaunch the interest in one game, if people come back frequently on one game they are most likely to spend more money in it and buy the sequels.
So they need hardcore paying players to stay, and people not paying in game content to go away. I don't really know how they achieve that.
On the other side, they need people to be satisfied enough in GW2 to buy GW3 too.