Phrasal verbs

Why are some phrasal verbs like they really are? I mean, I have realized that some of them are reduntand, I don't know very well, that's why I'm asking about it. (e.g.)Why is there sit down? Why "down"? Couldn't it be just sit? If you ask someone to sit, of course that it is "down". it can not be "up". What I really want to know is why does this kind of thing happen? CHEERS

[Tdol]

The are quite a few phrasal verbs where the preposition is unnecessary; maybe we're just addicted to them and can't resist adding one.