You then step to the South to meet with Clay, Cheren, and Hugh.  Clay applauds your battle and points out that the good showing should prompt more people to join in on the tournaments.

Surprisingly, Roxie shows up a moment later, asks if you and Hugh have any more 'crazy battles' to show, and doesn't bother waiting for an answer.  Instead, she declares her intention to join in and have some 'rockin' battles' herself for the sake of the excitement.  As she walks off, Clay takes this for a demonstration that he was right.  His intent, apparently, is to promote the tournament and develop everyone's skill so that people will come in from all over- and provide Driftveil with more money and growth.  With a few more encouraging words, he's off to the South.

Hugh is disappointed and complains that he wanted to win his first tournament, and Cheren will point out that the tournament is sure to draw in all the Unova Gym Leaders so that they can improve their various skills.  Colress can also be found in the lobby- he declares his intent to keep battling to draw out the skills and strength of trainers and their pokemon, so that his theory will be apparent to everyone.

If you like, you can turn in your lone BP for a vitamin (HP Up, Protein, Iron, Carbos, Calcium, or Zinc), but it will be much more valuable with more BP, as you can pick up evolutionary stones, unusual or highly useful hold items, and even a number of different TMs that aren't available through other methods.

Tournaments won't progress the story or gain you EXP, or even get you closer to becoming the Unova League Champion, though, so it's time to move on.

As you exit the tournament building, Cheren and Hugh both follow you, stopping to talk a bit.  Cheren says he barely recognized the two of you thanks to your growth since your first gym battles.