Neymar even has his tongue out like Michael Jordan when he does the heel control move.
Why not use your heel behind the standing leg to control the ball, that's why Neymar does. At first this just seems like Neymar is showing off, but he actually uses this heel control technique to push the ball forward and throw off defenders. Neymar uses his heel when he has time, haven't seen him do it when a lot of defenders are around.
Sure, he could just use the inside or instep of his foot to control the ball, but I'd argue this trick catches defenders off guard, while it also excites the crowd, too.
Watch Barcelona's Neymar use the inside of heel of his to control a long ball. He makes it look easy.
Here's another picture of the heel control move by Neymar.