I agree leaving it wet makes sense but there has to be some wierd chemical reason to allow it to dry right?
Homebrew use: Thoroughly wash all surfaces with detergent or a compatible cleaner, followed by a potable water rinse before application of sanitizing solution. Prepare a use solution of 1 oz. of Star San per 5 gallons of tap water. Apply on surfaces with a cloth mop, sponge, spray or by a 5 minute immersion. For spray applications, use a course mist, with pump or trigger spray. Spray 6 to 8 inches from surface; rub with a brush, cloth or sponge. With spray, cover or remove all food products. For all applications, allow to air dry, however surfaces must remain wet for at least one minute