Notable advantage in a lot of beginner and intermediate level sports. By the time you get beyond that everyone knows how to compensate for left handedness.
Easier for you to assault a castle with spiral staircases while using a sword.
I'm told that Ferniehurst Castle on the border between Scotland and England was built with the stairs spiralling in the opposite direction because so many of the clan that built it were left-handed.
May also have been a subtle way to help protect in case the castle was stormed. I've certainly heard of castles where the steps were sometimes irregular heights - those who lived there would be used to it, but invaders would not and would find it harder to move effectively. A differently spiralling stair might have the same effect.