Have you ever been punched by someone wearing ordinary boxing gloves? I have, and I find that the force behind the blow is just as great as that of a bare knuckled punch and the corresponding impact not significantly reduced by the inch or so of padding.
Why wear them, then? Make a fist, and notice how the knuckles of your middle and ring fingers protrude beyond the others (this assumes that you've never broken them). Were you to punch something hard, whether it be a wall, heavy bag, or an opponent's forehead, the impact would be concentrated in this area, very likely breaking one or two bones. Gloves spread the impact across your hand, reducing the chance of severe injury. They also protect against scrapes and cuts when practicing on a heavy bag.
Types of gloves range from the heavily padded "pillow" gloves sometimes used for sparring to simple thick leather mittens.