The basic function the expression you know serves in conversational discourse is said to be that of a pragmatic particle used when the speaker wants the addressee to accept as mutual knowledge (or at least be cooperative with respect to) the propositional content of his utterance. The fact that you know is even used when the addressee is assumed not to know what the speaker is talking about, suggests that it functions at the deference level of politeness, as a striving towards attaining a camaraderie relationship between speaker and hearer.