The purpose of having springs system is to eliminate the weight of the door so that the door will be able to be raised up and lowered down by hand or
by automatic motor.
When a spring is broken the door will feel a lot heavier if you try to lift by hand. The motor also will sense that the weight is too high and it won't be able
Properly adjusted springs should give you the ability to lift the door manually and stop it mid-way without falling down heavy or going up too hard.
There Is a range of spring sizes and lengths and the reason for it is simply because doors weight can vary depends on materials, insulation, windows etc.