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.

[nadiaines]

Actually you can "sit up". talking about a dog you would say "sit" to mean sit down, but the other way you would say "sit up".

you can "sit up": move into an upward position.
you can "sit someone up" move them into a sitting position when they have been lying down.
you can "sit up at night" i.e. you don't go to bed though it is very late.
sit up can also mean to pay attention to something.
in all these examples up is an adverb (particle)