The Malbim writes that Achashverosh emphasizes that Haman should do this, himself because Mordechai deserves these, and Haman does not. Therefore, as either a punishment or an opportunity to take part in honoring the king, he should get these items instead of asking servants as Haman (Esther 6:8-9) suggested.
Additionally, Me’am Loez points out that Haman is the trusted adviser of Achashverosh, so the king would prefer Haman handle the king’s precious items, and not servants with their grubby hands.
Perhaps Achashverosh, being the evil man he is, wants to lessen the reward given Mordechai by cutting out the honor implied by the presence of many servants.
Class Participant YML, however, pointed out that Haman represents the king, the source of this honor, more than do the servants. By Haman serving Mordechai directly, this would exponentially boost the honor Mordechai is receiving.