Is this a stunt for "cool points" or does it aid in stability/speed in some way (as in, helps you finish first)?
Typically cool points, but it can be used to align the bike with the landing surface if the launch, trajectory and landing aren't in a straight line.
Bar turns not only look stylish but can also aid in body movements on the bike. If you need to correct position for a landing, such as when you go over a hip, you'll need to turn the bars a bit.
As for performance, there is one time it will make a difference, and that's when you use it to 'scrub' a jump. This means it allows you to clear the jump with less height so that you come to the ground quicker and waste less time in the air.
But mostly it just looks really cool and generally indicates more skilled (or more fun) riders.