When a fire extinguisher is used, the pressurized gas within is released. Even if the pressure is slightly released by a small discharge, it may make it so that the extinguisher cannot properly discharge the fire fighting agent again when it is needed.
You can tell the pressure by looking at the gauges and seeing if the arrow is pointed to the green area. If the arrow is not pointed towards the green area, then your fire extinguisher needs to be recharged immediately so that it will be prepared to fight a fire in an emergency.