The tradition of a woman holding her husband's stirrup when he went off to war is part of Medieval chivalry - and thus sometimes shows up in Medieval fantasy (and, of course, in historicals).
However, most people have an image of the woman standing decorously at the horse's shoulder and turning the near side stirrup so the man can put his foot in it.
This is completely wrong - as any horseman knows. It's a great image - and it looks good to see her looking up at him with love - very romantic.
It's just not how you hold somebody's stirrup. A stirrup holder stands on the off side of the horse, firmly grasps the stirrup and pulls downward as the person mounts. If I'm mounting a tall horse from the ground, or a horse with no withers, I'll have somebody hold my stirrup. When teaching beginners, we normally hold the stirrup until they know how to get on correctly.
It's to prevent the saddle from slipping when all of the rider's weight is in the left stirrup. This makes mounting safer - and more comfortable for the horse.
But it's something a lot of people get wrong.