As a clutch is used, it wears just like brake pads do. Think about how, if you change your own brakes, you have to drive the piston back into the caliper (or spin down the self adjuster in a drum's case). That was the amount the piston adjusted itself to maintain the same gap between the pad and the rotor.
A non self adjusting clutch won't automatically advance itself as it wears, so as it gets older, it'll have to travel further to make contact. A self adjusting clutch will automatically advance itself to maintain a consistent gap while disengaged regardless of wear.