Garage doors are actually fairly heavy. By balancing the weight with a spring, you do a lot of favors to your mechanical advantage systems. (Eg: less wear, less advantage needed, the door can open in a reasonable amount of time…) You also do yourself a few favors for when things fail. (Eg: you pull the safety release and it’s possible to open the door without the motor.) Finally, power isn’t free and motors aren’t either; A spring is a really cheap way to reduce the cost of both of these things.