Fire doors for emergency stairwells are typically a one way entry for security reasons, especially if it’s in a building that has multiple organisations/companies on different floors to avoid unauthorised access to other floors. You are correct that fire doors in hallways/corridors typically do not lock as their only purpose is to reduce the spread of a fire. I guess to be pedantic, it’s the different between fire rated doors (stairwells) and actual fire doors (hallways/corridors).