A garage door can weigh 150 to 400 pounds, and the springs do almost all of the lifting. When a spring is wound, it stores enough energy to cause serious injury if a tool slips or a part fails. That is why R&Y never recommends winding or releasing springs without the right tools and training.
The other reason is fit. A spring that is slightly too strong or too weak makes the door drift, slam or strain the opener. Each R&Y replacement starts by weighing and measuring the door, so the new springs match it, and ends with a balance and auto-reverse test.
Have a Wayne Dalton door with an enclosed spring tube? That is a TorqueMaster system: see TorqueMaster spring repair & conversion.