Because (a) he was presenting his new positions on Iraq and (b) spouting Michael Moore-onic propaganda that he is not as familiar with and (c) to have a checklist to assure he got all his pre-programmed points across.
Remember, a few days before the debate, at NYU, Kerry debuted this new set of positions that makes up his "final" stance on the Iraq war. He needed to make sure he remained on topic since whenever he ventured away he made mistakes ala "global test".
Arrogance does not obey logical rules. Arrogant people are self-destructive, and often they share that destruction with those around them. Witness the Clintons (him and her).
Congressman Billybob