In fiction "as you know" and equivalent phrases appear before exposition which is needed for the audience to comprehend vital plot elements even though the characters in the scene really shouldn't need it reiterated. It can show up before a recap but it can also proceed and explanation of some in world conceit like how magic works or why they need a giant diamond for the super laser. As an expression it's kind of clunky and in isolation can cut into verisimilitude. For this reason it's best employed in a circumstance where the character being spoken to doesn't actually know. This can be because the character knows the facts but is too dim to draw conclusions from them, it can be because there knowledge is being tested, or it could be that they are drawing different conclusions from the facts.
In each of these cases the exchange provides some characterization on top of the exposition. Of course all of that also requires the author to come up with reasons why the characters have this particular dynamic and keep the story more or less consistent on that point. This is easier at the beginning of a story when there is no prior characterization for it to conflict with and gets harder as characters' understanding of what they are doing increases. For this reason episodic works that have to continually onboard new readers are often the worst offenders for deploying the phrase where there is absolutely no diegetic reason to rehearse the obvious.
"As you know" was most prevalent in mid Twentieth Century adventure genre pieces. Literally picking a random book off of faded pages I found four instances. Seven more books from the genre had three with one or more occurrences so I feel pretty confident from my tiny sample that this really was a thing. Interestingly, every instance was in a book from a numbered series. While I think it's easy to dismiss the phrase as a hackneyed cliche it's also worth considering that this is the sort of phrase real people can and do use when elaborating on the consequences or implications of some shared beliefs. It's only really a problem when it is shoehorned into dialogue which strains audience immersion.
IRON NODER XVI: MORE STUBBORN-HARD THAN HAMMER'D IRON