To standardize propellor design and manufacturing silly... In the old days props were started by hand and you wouldnt want some poor ******* walking under the prop to start it and have it back fire and hit him.. Here's one for you, I am an artilleryman btw.. NATO and US Cannons have a righthand twist in the bore and most soviet and others have a left hand twist.. Why? I guess Because cannon rounds when fired drift in the direction of the twist and this must be accounted for in deflection to hit the target..