Why transfer vassals? (Crusader Kings 3)

For example: If you transfer the title of duchy to your son, then all counties in this duchy de jure belong to him too. Try to match de facto and de jure. Then there will be fewer internal conflicts.
When a vassal is transferred, the title remains with the same person, but he is no longer directly subordinate to you, but to the one to whom it was transferred. If before that there was an inscription "your vassal", then it will become "a vassal from your state"

Crazy Garik
A person cannot be in two places at the same time.
The king needs helpers to control the land effectively, and he negotiates with other people on mutually beneficial terms. With them, control over the land and the flow of gold are preserved.
Power vertical!