Why are some closets made to lock from the inside?

My house is about 100 years old, and all of the closets that still have their original doors from when we moved in lock from the inside. There is just a doorknob on the outside and the inside doesn't even have a knob, just a lock. There is no slot for a key on either side. What is the purpose of this? Was this just some sort of bad doorknob installation or did this actually have a reason?