6x9 speakers allow you to fit a larger speaker than space allows. They're very common to see in cars with rear decks (below the rear windshield). If you didn't use a 6x9 there, the other option is a 6" or 6.5" round speaker. The area of the speaker cone is considerably less than a 6x9. The result of which is the 6x9 can provide better bass response due to it's larger size. We really like 6x9's whenever possible. There aren't as many choices, but they are a great size for rear deck's in cars.