Why do 24-hour convenience stores have locks on their doors?

Interesting question. 

According to their working hours, these stores should never be closed. 

But in which case the doors can be locked?

First of all, there is a human factor. Every store is runned by people, so there can be some situations when they need to leave the store for some time: visit WC or something.

Second, in case of robbery or some need to protect the store the doors can be locked.

Third, there is a special rule for the workers. They usually work for 7-12 hours and then the second worker comes to change the first one. If the second worker doesn't come, the first one has the right to leave but not forget to lock the door.

(i've decided to do another entry for today)