What would that be?
Simple!
You have like twenty bronze cards (Vassili) and you prefer 5 of them and you don’t want to accidently trade one of this 5 cards. So you LOCK it and wont be able to trade those 5 locked cards. If you want to, you have to unlock first.
By my experience, I have 20 Lead~Iron Cards and I was using a Iron Skyhammer that I really liked (my first opened case and his stats was good) and I accidently trade him to make other cards and I lost him 
So, I’m sad and I hope my suggestion is accepted. I really don’t see it as a big thing, but could help. 