Pneumatic Actuators, materials used for piston and cylinders?

Hi, i need some serious enlightening for the subject mention or rather pertaining to it. I know what's the working principle but my lecturer posed a totally unexpected question. Why are the piston fabricated from steel chromed plates and cast irons for the cylinders? Why can't the surface of the piston be dull rather than shiny? Is there a logical explanation to it or it's just for aesthetic reason? Please advise. Thanks