Because it makes the turn prettier, and it lets you trim the plane out to fly level easier while still achieving tight turns. These are entirely different reasons than the usual ones for putting flaps on a plane -- usually you put flaps on an airplane to give it more lift at high angles of attack, when it is going slow. Here we're putting flaps on a plane to give it controlled lift at zero or moderate angles of attack when it's going fast.
Flaps make the turns prettier because with flaps the wings can develop lift at a lower (or even negative) angle of attack. Without flaps the wing has to be at high angles of attack, which means that the tail swings out of the corner -- it looks like the thing is fish-tailing. With flaps you can adjust things so that the plane looks like it is traveling on rails.
I'm not entirely sure why it does the level flight thing -- I think it's because the you can put the center of gravity further forward. Expert-er folks than me can opine on this one.
If he's seriously interested in control line stunt, and not just noodling around, he may do better with a SkyRay or a Flight Streak. They're way easier to keep flying after crashes, and you won't be distracted by the complexity of tuning a flapped ship. I was given this advise by Brett Buck when I was starting out a few years ago, I followed it, and I feel it was an assist to getting me up to where I am now. Once you know the pattern and you aren't crashing regularly, then move to a flapped stunter.