It varies a little bit between munitions but usually police are only supposed to aim at limbs with them from more than a few meters away. They're intended to be shot at people's extremeties to force them to drop a weapon so that they can be subdued (and provided with neccessary medical care shortly after) without hurting themselves or someone else. For instance if someone is threatening officers with a knife, police could shoot them in the arm, likely breaking the bone, and forcing the person to drop the knife.

Getting hit in the head or center mass with them often leads to life threatening injuries because they break bones (like the skull or ribs), cause internal bleeding, and bruise or rupture organs. If they're being used for crowd control they should only be getting shot at legs or feet from a substantial distance and even that is extreme.