As others have said, Alaska Airlines has a big presence in Seattle, plus many mileage partner airlines (basically, both the US-domestic One World airline (American) and Sky Team airlines (Continental, Delta, Northwest, plus various other airlines).

If some other airline (particularly a Star Alliance airline like United, US Airways, or Air Canada) matches up with your flying better, you may still want to have an Alaska Airlines account to collect miscellaneous miles for when you primary airline's flights are not suitable.