Also the dimples are small enough to allow the air to "skip" over the holes, but not be disrupted by them so the air does not create drag in those spots and overall acts as if there is less surface to have drag on. Even a smooth surface generates drag, but a dimple is as if that spot wasnt even there.

Also I seem to recall about 7 years ago I think it was mercedes that employed a full dimpled plate to the undercarriage of a car for drag reasons.