They're mostly there to serve as the source of truth, whether or not a specific manager is getting a bonus (and by how much).
Generally, the best metrics are "based in reality" (and usually revolve around inflow and outflow of money) -- but those are hard to fudge, without playing accounting games, and even harder to meet (it takes actual skill, rather than being able to pass the buck endlessly for a couple of years, before moving onto another org); so you may find it common for the managerial horde to pick "bullshit metrics" to cover their own asses (doubly so, if the metrics are decided by committee).
There's no reason for practical metrics to exist in most large corporations -- the incentives just aren't aligned. Bonuses are decided by metrics (and anyone who has a modicum of intuition, will always pick the metric that can be most gamed for himself, and his underlings). Also, if we're being real with ourselves, there's very little that managers "control," so picking a "real" metric would just breed an environment that selects for luck (or lies).
In my opinion, the only useful metric is profit generated. Everything else is just an emotional safety blanket for uncertainty.