xD I love how Discowolf and I said the same that at the same time. And yeah.

They are helpful with mobs as well. But they don't attack creepers I can understand why.

Ok cool

Actually, if you attack a sheep for example (with just your hand), the dog will go and hunt down that sheep (or any animal) until it's dead. Also does the same thing with players in SMP. Also good for guarding houses, make them sit by your door and it will attack anything that comes near. (Except players).

That's my take on it.

Yours truly:
Honey_badger456

If you mean MC dogs then what they do is when you punch somethings they will start attacking it