If you withdrew some and bought a present for the missus, does that still count as part of your bankroll?
Let's just be clear, you wouldn't actually take the present back, but would it entitle you to take the same from your own bank account if necessary?
If you view your bankroll as a total virtual amount spread over different places, how easy or difficult would it be to get it to where you need it, when you need it?
Managing several separate rolls in different places is certainly easier.
Of course it is up to the individual how they want to view and manage things, and also their abilities for more complex management arrangements.
I would say though that it is probably a case of;
If you need to ask the question then it probably isn't for you.