In many large buildings (schools, etc. in particular), you want the internal fire doors to close to keep the fire from spreading. Of course, exits must be provided for each isolated section.
It prevents the fire from quickly moving through the building...
Also, people.
Yeah, that's a given, but the point of fire doors is to maximize overall survivability instead of the "average doors crossed per minute" stat in isolation.
The doors don't lock, they just close.
These are installed on doors that have an auto-closer anyway. They deter people from propping them open permanently with a wedge or a chair.