The 'you know' is a filler we use for traffic regulation. We use these fillers subconsciously to take micro breaks while speaking, to verbalize our thoughts and to indicate to the listener not to interrupt as we aren't done speaking yet.

Polish of delivery is just a nice to have feature, substance matters most of all.

I've attended public speaking courses, which tend to emphasize polish, but seeing some of the most effective speakers using fillers like 'ums' 'ahs' and 'you know' just goes to show polish doesn't matter all that much. Reality supersedes theory.