2008 Toyota Tundra Timing Belt

When replacing a timing belt on some vehicles, why do you need to mark the old timing belt and match the marks on the new timing belt? I was watching some videos and it seems like some mechanics not only line up the marks from the cam(s) and crankshaft but also the belt. I don't understand why some mark the timing belt itself, is there a different teeth on the belt that lines up on the cam or crank?

Thanks,
Eric.