I remember learning about this in my teaching classes in college. Saying "Because I said so" doesn't teach the child anything but that you have dominance over them.
I agree with Rob Winters who said that sometimes people/parents use it as a short answer or as an answer for a child who may not understand the meaning behind the action that is wanted. It can be frustrating at times to deal with a child who is challenging a request made by a parent or teacher or other adults.
I find myself saying that to my son who is turning three, but then I remember my college days and tell him the reason in a story of sorts, kind of like an extended metaphor for what I'm asking him to do. I bring the reason down to his level; it may be in the form of a nursery rhyme or an example from one of his favorite books. Does he always understand? Perhaps not, but the story does divert his attention and stubborness for a minute and he most often complies with the request. :)