Garage doors are not something the typical homeowner tends to think about on a daily basis, it's little more than a door that goes up or down when you push a button. Maybe you park a car in the garage, some people use the space for a craft or hobby, perhaps the garage is a place for a motorcycle, boat, or antique car to be stored during the winter. What you will notice is when the garage door doesn't work as expected and you realize how much is going on with the mechanics of how the door works and how important proper maintenance is to assure continued performance.
The mechanical functions of your garage door allow for the motor which raises and lowers the door to function efficiently without using more power than necessary. Springs provide tension so the door can go up and down safely rather than simply slamming itself upward and falling down uncontrolled. Cables and chains direct the force provided by the motor to the door which works in tandem with the springs to raise or lower it. The rails guide the door so it stays where it was designed to be and you can rest assured it will remain in its proper place rather than having to keep a foot of space clear on either side of where the door is supposed to be. When these mechanical features get out of alignment, you'll understand how important they are to the overall design of the door and its proper function.
In addition to the mechanical function of the garage door, there are safety features which can come out of alignment and prevent the door from operating the way it should. If the door encounters any resistance as it lowers, it will automatically stop and return to the raised position. Usually, it's going to be a bicycle or car parked in the doorway and you'll be glad it wasn't damaged by the door continuing to close on it, but more importantly is when you try to close the door and didn't realize a pet was in the way. A secondary safety feature is a laser pointer which reflects back on itself when nothing is in the way. It does get out of alignment sometimes and needs to be adjusted, other times debris may block it and signal for the door not to close. It may seem like a nuisance, but you'll be glad to have it when you need it.