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I noticed that my side door is equipped with two locks. The upper lock is a single-cylinder deadbolt, while the lower one is a lever lock. Both locks are operated using the same key.
So what's the purpose of the up one? It seems redundant?
Outside:
Inside:
This is an extremely common setup.
The top lock is a deadbolt. A deadbolt, within the limits of design (some types of locks are easier to pick than others) and installation (a lock is in a weak door or a weak frame then it will only be as secure as the weakest part) is generally very secure. In general, getting past a locked deadbolt requires some violent action - breaking the glass to reach in and turn the latch (which is actually a very real problem with glass doors or with doors that have a glass panel next to the door, but for fire safety you don't want to have a double-cylinder (key needed inside or out)) or cutting into the door or frame.
The bottom lock is, well, not much of a lock. It doesn't provide much security. It is enough to keep pets and (friendly) kids out. But it does very little to stop a determined adult. It is often trivially easy to pop open the lock with a screwdriver or sometimes even just a credit card. A telephone guy I worked with once referred to a flat screwdriver as the "universal key" because it works quite well to get into any telephone closet that doesn't have a deadbolt. In fact, I actually recommend disabling the lock mechanism on this type of lock, particularly if it is designed such that you can accidentally lock yourself out. With a typical (there may be exceptions) deadbolt you can't lock yourself out. But with a lock like this you can lock yourself out while at the same time doing little to prevent the bad guys from getting in.
In other words - use the top lock for security, and use the bottom lock just for the handle to open/close the door.
From a locksmith's perspective, you have a deadbolt lock and a spring latch. While the latter does have a locking knob and a deadlocking latch, it is considered primarily a convenience device; the deadbolt is your real security. This is a very standard combination.
Yes, there are places where the knobset alone is considered adequate security. If you live in such a place, you might want to use the deadbolt only at night, or only when the house is unoccupied, or at other times of increased concern. I know lots of folks who take that approach. But again from a locksmithing point of view, there are advantages to being just a bit more secure than your neighbors, so a burglar is encouraged to seek an easier target... so I do use the deadbolt. It's not that much inconvenience once you get used to it.
The purpose is to give mental reassurance to the person inside that the door is secure and they are safe. Some individuals go to great lengths to get that sense of security.
