What is "You know" in many sentence?

It can have several meanings :

1. To introduce a topic of conversation : You know that shop on the corner of Oaktree Road? Well, apparently it's closing down.

2. As a hesitation device to give the speaker time to think about what s/he wants to say : I was well, you know, a bit surprised because he was, you know, not the person I'd expected to see. This can be very annoying if the person uses it too often - I once saw an interview with Prince William where he seemed to it every five or six words. like is often used in the same way - He was, like, not the person I expected to see.

3. With rising intonation, a way of checking that someone has been listening an following what you've said : I was surprised, because he was not the person I expected to see. You know?

4. To suggest complicity and shared information between speaker and listener : Well, you know what he's like.

Notice that in 1 and 3 it's actually a question, but question form (Do you know...?) is not used. This is because the speaker assumes that the listener does know. It's not a "genuine" question - the speaker is just asking for cobfirmation that the listener understands what she's talking about.

I hate how people use "you know" very frequently, just as others would use "uhm" or "ah" or "like." Why can't they talk in direct-to-the-point sentences?

Susan53 has a great explanation.

I would only add that what you are probably hearing most of the time is her #2 and #3.

The hesitation with "you know" is very common.

Also, checking that the listener understood, or agrees, is very popular in conversation as well.

Words like "you know" , "Um' "Uh" are often used as conversation fillers. A filler is usually a word or two that is meant to give the speaker time to assemble his or her thoughts before speaking. They don't usually mean much.
We have others like "Well" especially used when stating an opinion after a question is asked.
Fillers simply buy us a bit of time. And they exist in all the four languages I speak - French, English, Chinese & West African Pidgin English. I guess they do in every language.