Almost every four legged pet gets out at some point. Nobody expects it, but it happens. Other than the obvious dangers from traffic, a couple of things can happen when they are on their own.
Most often, a stranger finds them. Without any tag, they have few options. They may keep your pet, but usually they call Animal Services. Your pet will be transported to the pound. If you know how to check at the pound, and you do find your pet, open your wallet. The fines and charges are significant.
If you happen to be out of town for a week, and your house sitter doesn't check the shelter, your pet may be adopted after the four day hold period. Or if the shelter is full, and your pet looks less desirable, it might be euthanized.
Now let's suppose your pet has a tag with your phone number on it. That neighbor calls your house, and your pet is home. No fines, fees, dangers or death.
If picked up by Animal Control, you will get a call. Often your pet will be dropped of at your house. At worst, you can pick up your pet within a day, and fees and fines are minimized.
Don't want to use collars and tags? Microchips work. $30.00 at Animal Services. Every animal taken to the shelter (or a vet's office) is scanned and you will still get that call.
Think of this as very cheap insurance. A couple of bucks for an ID tag with your phone number, or a microchip, can save you hundreds of dollars or your pet's life.
If you love 'em, tag 'em.