Portable fire protection appliances commonly known as "Fire Extinguishers", are a means of dealing with a small fire, to prevent the fire from spreading. They should be used by a competent person and only under the right circumstances and with extreme caution.
There may be an instance, where perhaps you, your colleagues, or your family are trapped on a stair-well, or the emergency exit is blocked by flames, or perhaps the blockage is caused by something else, therefore you have to use the fire extinguisher to tackle the flames, in order that you can utilise the blocked exit, or perhaps reach an un-obstructed emergency exit.
A fire extinguisher should not be a means of tackling a large fire. Faced with this situation, one should call the fire brigade and follow the evacuation procedure, meeting at the designated assembly point and completeing a roll-call or register to ensure no one is left behind in the building which is ablaze.