"Clothes make the man" was first expressed by Homer in The Odyssey and repeated by Erasmus, Shakespeare, and Mark Twain. Like it or not, clothes matter. Kellerman uses them as a tool to achieve an end. He allows Andrew to bond with the client before cladding Andrew in new armor.
It's only the armor. The dossier is the sword.
After the victory, Kellerman becomes the voice of temperance urging him to use power sparingly or he will become the monster.
The win gives Andrew the impetus to go after Will.