To be on the correct lead will keep yourself and the horse balanced, however "counter-canter" (eg. The horse is on the left lead whilst going clockwise on the circle) can be deliberately asked by the rider the horse should bend with the poll to the outside, this can improve coordination and communication (this is different to the wrong lead because you have asked for it).
I hope this helps😊
It's a balance thing. When a horse is on the correct lead he can correctly balance himself around corners and durning circles. When they are on the correct lead it can make things very difficult for the horse.