Pre-Order for Early Beta Access
There is an open Call of Duty: Modern Warfare beta, so you can try it out without pre-ordering, but if you pre-order you can play early.
Activision confirmed two weekends for the beta, with two extra days of play for each of the weekends if you pre-order the game.
This can double the time you have to play the Modern Warfare beta. That's worth pre-ordering for many gamers.
While progress doesn't carry over to the full release, the skills you learn and map knowledge you pick up will carry over.