Okay, this is going to be a pretty broad answer.
Your key word is unplanned.
Let's think about it this way...
You're having a conversation to your best friend about your summer holidays (aside from study). Surely, some things here would be implied (thus your "you know"s), some unplanned (thus your false starts), even some not completely thought out ideas (thus your discourse particles, "um"s and "ah"s). You don't really practice it like you would an assessed speech in front of your Eng Lang class, right?
Unplanned texts are thought of simultaneously and sometimes, your mouth is faster than your mind. After all it's doing several things at once - recalling your (hopefully happy) memories of the summer, moving your mouth up and down, etc. (You didn't get a chance to make this stuff up beforehand - it's unplanned.)
To counteract this (and so not to create this awkward silence in a *supposedly* fluent conversation), we use discourse particles to cover up this space and let ourselves think ideas and words through - we just need a little time.
tl;dr: We're human, we all need time to think ideas through while your mouth motors on, so we have discourse particles to keep the conversation flowing.
Please feel free to correct me.