Garage door springs provide additional lift and control for a garage door. Garage doors typically weight more than 150lbs so without some extra help, they are hard to lift manually or with an opener. There are two types of springs used in garage doors: torsion springs and extension springs.
Torsion springs are pre-wound or coiled up by a certified garage door tech so they have some force stored in them. They naturally want to unwind but the weight of the door keeps them from unwinding. But when some upward force is applied to a closed door, that energy in the spring plus the applied force allows the door to open with minimal effort.
Extension springs do the same thing except instead of winding and unwinding, they stretch and compress like a slinky toy. When the door is closed, the spring is stretched out and want to compress but the weight of the door keeps it balanced. A small applied force in addition to the stored energy in the stretched spring will allow the door to be opened.
In both cases, as the door is opened, the door weight is transferred to the horizontal tracks which means less downward force. As the door closes, more weight is transferred vertically while the springs provide more and more upward force, thus preventing the door from crashing down.
In summary, the springs provide additional force to maintain a balance between the weight of the door and the energy stored in the springs.