Suppose you're a mage doing heroic rag on trap duty. Being able to quickly see the status of the raid:
- Is there somebody low health likely to get killed if I break the trap now
- Do the tanks have cooldowns up to handle the burst damage?
- Are the healers really low mana and unlikely to be able to heal up the damage before transition?
- Does anybody have the trap damage debuff (maybe you blocked it off or got ressed because you screwed up).
Suppose your a hunter tunneling down exploding spiders on beth, is your healer out of range? If so, which way do you have to move to fix that. Can you call for pain suppression or is the drone tank about to die? Imagine you're a deathknight on the same encounter beating up drones: is the hunter soaking the spiders dead and if so, where do you have to go to res them? Maybe you can't res and nor can anybody else: should you take over their job or has a moonkin already picked up the slack? Maybe they just fell behind and the spiderlings are a bit closer than normal but otherwise everything is fine. Grid gives you information to figure out what the heck is going on so you can act appropriately.
Maybe you're a mage beating up seeds on rag: which players is getting their butt kicked and could probably be helped out with a frost nova before world in flames happens? The low health person might be a good choice. Maybe you're a ret paladin and can save somebody with a hand of sacrifice, or you're doing sinestra and you see somebody dazed by whelps that can be saved from cutters with a hand of freedom. Maybe you're a fury warrior and you see somebody getting low because they herped up tornados on alysrazor: you'll know it's a good time to use rallying cry to save them. Perhaps you're playing a rogue and your normal tricks target is dead or out of range, maybe a warlock or shadow priest just got 150 stacks of stolen power on nefarian: you're much better off tossing tricks on them than on a rogue that just lost their stacks.
Knowledge about the status of your team mates is important. You don't think football players are constantly scanning the field not just for the opposing team but also their buddies even though only the quarterback is going to be tossing the ball? I think a better question is "how can so many pure DPS classes find room in their UI for a damage meter but not for a grid-like addon?" Grid isn't a healing addon: it's an information dense picture of your group's status.That sort of information is valuable for any player: tank, dps, or healer.