What are airplane propellers used for?

They are used to propel the airplane forward to get air moving across the wings. When air moves across the wings lift is created and you take off. Another interesting note: A propeller is an airfoil just like a wing is.

They make the airplane more versatile, less expensive, or both. A similar question is, "now that they have jet airplanes, why do they make propeller planes at all?" First, a propeller plane is less expensive than a jet. Let's give you six million dollars to buy an airplane, and make you buy new. $6 million gets you an entry-level jet. It will carry four executives if they restrict themselves to one suitcase each. The same $6 million will get you a Pilatus PC-12 or a Beech King Air B200, which are basically small airliners, or it will get you two EADS Socata TBM 850s, three Cessna Caravans, or three Piper Meridians. And that's just turboprops--$6 million will get you about a dozen piston planes. The other reason, and this is more important for a lot of operators, is that the propeller plane is more versatile than a jet. You can land on unimproved airstrips. You can land on top of mountains. In Mel Gibson's movie Air America there's a scene where he lands a Pilatus Porter on the side of a mountain--the real Air America pilots in Southeast Asia did that a lot. Jets, unless they're built for the military and equipped for rough airfield service, have to fly onto paved airstrips. Airbus is busy designing the A400 military transport. It has four turboprops. The drawback of a prop plane is that it can't be very fast--if you need a plane that flies faster than 400 knots, you need a jet. Prop planes don't fly as high as jets do; a King Air will fly up to 31,000 feet above sea level, while business jets routinely ply the skies above 41,000 feet. Propellers are not used to force air across the wing. they are used to create motion. In other word make the plane move and it's the resultant forward movement that moves air across the wing to create lift and get the plane off the ground