It's a legal requirement here in the UK to have a collar on your dog.
To be honest, I don't know why people wouldn't want to have a collar on their dog along with their contact details.
There have been times when our family dog has somehow become separated from us whilst out walking and has run back to our car. Our mobile number is on the collar so that if someone finds her, they can call us to let us know they have her and where she is. Thankfully it's very rarely that this happens but we have also come across dogs that have lost their owners and have been able to call them to let them know we found their dog.
