That’s the built in leaf blower to make sure that the runway is clear for autumn landings. Most airports have their own de-leafing crews, but not all of them.
They were mandated to be installed after a string of landing crashes in the 90s got people making “fall” jokes too often, which upset the FAA.
It act like the small training wheels in bikes. New pilots struggling with those big noisy engines put this small training propeller and feel confident.
It’s the auxiliary retractable engine, for STOL operations. It gives the aircraft turboprop like performance in takeoff and landing, with no detrimental effect on cruise performance.