Fire doors with panic bars may have OAD handles/knobs on the outside to allow the door to also be used an an entrance.
Doors fitted with older OAD such as the Exidor 298 cannot easily be re-keyed if the tenants change or keys are lost. (The 298 is also discontinued and nigh on impossible to find). In the photo above a new style Exidor knobset has been fitted that has an easily replaceable lock cylinder.