It's true, meditation is basically a charge bar for certain spells. These being - Delayed explosion, Elemental shot and elemental barrier. That's all i'm aware of at the moment. That's why sometimes you have to double cast a spell which requires meditation, as when you cast it the first time, it'll give you the minimal charge to get the shot off once.
Obviously, the higher the charge the higher the chance of hitting a higher damaged shot. Higher.