The rated delivery of a compressor was measured on a new machine. Worn piston rings, tired valves or a slipping belt all take their share, and nothing on the compressor tells you how much.
A timed fill does. Fill a cylinder, note the time at regular pressure marks, and the calculator turns that into litres of free air per minute. It allows for air’s real-gas behaviour and the cylinder warming up, and shows the simple pressure-times-volume estimate and, if you enter it, the rated delivery beside it.