For example, let's say I'm a King with two Kingdom titles. I'd like to give one of them away to my relative; and I would like all the vassals under that Kingdom de jure to go with it. The issue is that if one of said vassals owns even one province outside the de jure Kingdom, they stick with my Kingdom. I can't pre-Grant Vassal them away because they're vassals of the same rank, and it would be very nice if you could select which vassals go with the Kingdom.
As an illustration, here's an example from my current game. I'm trying to give away the Kingdom of Estonia, but it won't transfer the owner of Laanemaa, Livs, Tartuu, and Vidzeme, just because their owner also owns Talava, de-jure part of the Kingdom of Lithuania (which doesn't even exist). I can't give Chieftan Idley here the Kingdom and then grant vassal because I am also a King, and he will become independent the instant I grant it.
As an illustration, here's an example from my current game. I'm trying to give away the Kingdom of Estonia, but it won't transfer the owner of Laanemaa, Livs, Tartuu, and Vidzeme, just because their owner also owns Talava, de-jure part of the Kingdom of Lithuania (which doesn't even exist). I can't give Chieftan Idley here the Kingdom and then grant vassal because I am also a King, and he will become independent the instant I grant it.