I think the judges ask them to do a dance that they want to give 10s to.
Sometimes that's the dance that they had a small error on. Sometimes it's their first dance to see improvement, sometimes it's one of their crowd pleasers that was perceived under marked.
The point of it is to be able to chuck 10s at them.
I actually think J&As Paso or Waltz would fit better as a judges choice (assuming if they get to the final their Jive is their fave dance) but I can equally see a case for the redemption storyline for the QS.