As an American Idol fan, it is neat to see the basic layout of the live shows that you see on television.
Additionally, guests who win the final show of the day also have a chance to tryout for the actual American Idol show for the next season, so there is a chance that you might see someone that day that could make it to the show.
Also, be on the lookout after each season of American Idol because you never know who you might bump into from the show. A couple of years ago, my wife and I were at Disney for our honeymoon and walked right by a couple of the top 10 finalist from 2011!!