I'm guessing you mean the ones used when people want to convey something. I don't really know how it came to, or when it came from, but I think "you know" was shortened from the sentence, "do you know!?", and it was used to strengthen the meaning on what people say, that's why I add that exclamation mark. If you're a Japanese, it's like saying: "desu yo"

It's a habit that people develop. I've seen it where people start each sentence with "you know". It makes no difference to the sentence being spoken, and is never used when writing. I've also seen similar usage of " obviously " or "basically".

Well, I'm not sure, but I guess it serves as a spoken/ informal word for resumption (I don't know other word to describe the statement, any corrections are welcome!). For example: I can't go out with you this Monday Chika, you know, I have to study :'( Hope I made it clear! All the best,

usually whether its in the beginning or the end of the sentence you are asking or telling the person to understand your feeling and that they have probably shared the same feeling (A) i hate it when people park over the lines, you know? (B) yeah totally (A) you know, its like hes my friend but I dont want to pay for him (B) Yeah I understand completely I think its most comparable to ね in japanese. It invites a short response of agreement

Thanks!