I think Casselden uses "are you quite sure of that" as a tactic to stop RB later changing his story.
For example, when RB says something that Casselden knows is a lie, he then gives RB the opportunity to correct the lie, but RB doesn't, he just further cements the lie, essentially saying the lie multiple times.
This way, it will be hard for RB to change his story or argue he was being pressured and his words twisted.
Casselden used it several times today, when he read RB's testimony back to him and called him out for changing a story despite being asked earlier, "think carefully, are you sure of that, are you quite sure?"
Either that or the court is keeping score of every time he perjures himself and they will fine/charge him for every single lie, so they see getting him to repeat lies to double the penalty, hahaha.