Fire extinguishers are self-explanatory -- if there's a fire, you extinguish it. But when these amateur (and in some case actual) firemen bust out their tools, it's usually to keep the fire of mischief roaring. In case of boredom, break glass and watch this video.
Firemen aren't the only important members of society that need to cut loose. If our folks in uniform, fighting the good fight overseas and at home, want to goof around for a second, we say at ease, soldier.