The springs are wound up to assist in lifting the weight of the door.
If you add lots of insulation to your garage door, it will cause the door to be heavier and the springs had to be tightened up with more turns. Well, when the weight was removed, the door would not stay closed.
Countless people have been KILLED trying to work on springs on garage doors.
PAY FOR THE DAMN PROFESSIONAL GARAGE DOOR GUY, they have tools and they know, how to do this job safer then you could.
You can watch the guy the whole time and ask questions, you will think you could do the job, but you do not want to be that guy when the springs pop loose.
The springs are used to assist the garage door in lifting up the door. Without the springs the garage door opener would need to be stronger and the substructure holding it wound need to be seriously strengthened to support it while it is in use. With things like the springs and a lower-powered garage door opener it makes it cheaper and easier to own, maintain and install a garage door opener is almost any garage.
Because a garage door is very heavy. You would probably not be able to lift it without the help of the springs. The springs act somewhat like a counterweight, applying force to the garage door, which reduces the amount of force needed to open the door.
The springs also help to keep the garage door from crashing down with excessive speed and force, which is likely if there were no springs to apply force in the opposite direction.