Why did the first Marine uniforms of the US military include high leather collars?

It orginated with the earliest Marines, in the Revolutionary war era. From the start, the Marines have been amphibious in nature, and many of their battles involved boarding British warships with cutlasses in hand. Thinking ahead they wisely wore a thick leather band around their necks to prevent unnecessary injury by enemy swords. Hence the nickname 'Leathernecks'.

very good answer.. thanks!