description of fire extinguisher

I saw many fire extinguisher around my faculty.I saw many of it was placed in the corner on floor. That are red in colour. That have a key above the fire extinguisher. That placed and locked in a box but i din't understand why it locked into the box. If somethings bad happened, i think it will delayed the firemans to play their role for rescue people. Ands i saw some instruction in it to show how to use the fire extinguisher. It quite useful for someone who don't know how to use it. I think evryone should have it because it is very useful. For example, we can use to hit the thiefs. Haha.