Why do some power adapters output an AC voltage?
One reason is that you usually need positive and negative voltage for audio amplifier. (Like +5, 0, -5 V).

Another reason: Some devices run directly on AC, e.g. Christmas light bulbs can do.

Yet another reason: Some devices need two different voltages, like 5 V and 12 V.

Yet another reason: An external step down transformer provides just safety. Voltages no higher than 50 V are usually accepted as safe (although not very safe with high current). So the transformer makes it safe, the rest is in the device.