Many people take their garage door for granted, using it as the sole entrance to their home and expecting it to work every time. People get so used to the door opening every time they push the button that they stop carrying the keys to their house with them, choosing instead to rely on the garage door as the only way they can get in. The issues happen that time when you push the button and the door doesn't go up, due to one of the many moving parts of the lifter mechanism wearing out. You came home after a long day and pushed the button and nothing happens. You check the batteries and they are fine. If something has gone wrong with the mechanics of the lifter mechanism, there is no way to lift the door from the outside. Even if there is a lock on the door itself, you would still need to disengage the chain in order to lift it. So now you are sitting outside with no way into your home. The lesson to be learned is to always have a spare key to the house somewhere where you can access it in this type of situation.
The other issue is that once you are in your home and have released the chain so that you can operate the door manually, now you cannot lock it. Most doors equipped with lifters do not have locks installed on them, and when the mechanisms are not working the lock is not working either. While you can still easily lock your interior door, the whole time that the lifter mechanism is broken everything in your garage is at risk of being stolen. Even worse is that anyone who opens the door and closes it behind them can work on your interior door without being seen. This is a big problem that can only be fixed by either keeping the door chain engaged and leaving your house through the front door, while parking in the driveway, or getting the door fixed as soon as possible. Let's face it, these are moving parts and they wear out. There is a plastic sprocket turning a tight chain to lift a heavy door. Anything under that kind of stress is going to break sooner or later, so isn't it far better to be prepared before it happens than learning a lesson after it does?
One of the best things you can do is have a qualified garage door repair person come out to look over your door periodically. They can look for signs of wear and replace anything that needs to be replaced before it breaks. If you wait until after it has broken you are going to be very inconvenienced and actually in danger until the repair happens. A little periodic maintenance can avoid these types of situations, and save you a lot of grief.