theres a drain plug, usually a screw, at the bottom of the tank that you let the air out of after every use, it's due to the natural air moisture collects at the bottom of the tank and unless you drain it after every use it will rust the tank!!!

every use is a bit excessive, depends how long/frequency you use it, in the bodyshop it's drained every night, however mine at home which i use quite alot is every sunday