I've been working my way through episodes of Babylon 5 over the past year. I'm now coming towards the end of season 5 - and I noticed something that hadn't caught my eye before. President Sheridan is signing a sheaf of papers before a meeting - and he's left-handed.
It reminded me that I had once belonged to a book discussion forum where there was a list being made of left-handed characters in fiction - but it's long enough ago that I can't remember what the forum was called.
I do know that I contributed to the list. One of my favourite characters in children's fiction was Drem, in Warrior Scarlet by Rosemary Sutcliff. He's a Bronze Age boy with a withered right arm, and therefore does everything left-handed.
I seem to remember that there was a character in one of the Darkover books by Marion Zimmer Bradley who was left handed, too - but she wrote so many Darkover novels that I'm not sure where I saw it now.
I'm not sure how much of a spoiler it is to say that a major character in Game of Thrones is also forced to become left-handed - but after that, my mind goes blank. The list I remember had a lot more characters on it than this, but googling has so far not been helpful.
In my own fiction, I wanted there to be a genetic component to the magical powers that some of my characters have, and also for the powers to be fairly rare, so I linked the powers to left-handedness. The characters are also allergic to cold iron, which gives them a disadvantage to go with their magical advantages over ordinary mortals.