Many voters also want to support the winner. If the polls show that one candidate has a significant advantage over the other, the perception grows that the winner has already been chosen. Might as well go bowling instead of bothering to vote. If the race is close enough, the head to head television debates that will inevitably take place can be very important. Debates are the one facet of American presidential politics that almost everybody gets a kick out of.
Watchers do get to see famous people squirm under intense pressure.
It is very hard to WIN a debate. It is very easy to LOSE one. John Kennedy won the closest election in modern history in 1960 because he looked better than Richard Nixon. In 1976, Gerald Ford sounded like a dope when debating Jimmy Carter. In 1980, any chance Carter had of beating off Ronald Reagan died in a lackluster debate performance.