I do it to fill in time, alts I only want to take "so far", so eventually it all end up back finding things to do on the main.
Almost out of non-PvP achievements to get, and I don't enjoy pvp enough to grind them so I'll be happy when all the tabs bar pvp are completed.
I've changed mains a couple of times - and have had different chars for raiding than the one I considered my main. But I have passed 10k on my cata main now (and think I'll get another 1-2k with account wide achieves). I think achieves are great for giving me something to do other than run laps of Org/SW that actually have something tangible at the end.
To the comment why not go do something IRL - if I'm only playing for 1-2 hours when I do play, and it isn't on a raid night it doesn't mean I should log off. Achieves give me something to do. Personally I don't run LFR (get all the raiding I need within guild and don't care/need the tiny upgrades for alts - alt raid will get there) and endless LFG is horrible...