I believe it shouldn't be "zero", but a user-defined threshold.
My budget calculation is largely fixed or sufficiently similar each month (Rent, transport pass, insurance, certain amount for food, etc.), so I would much appreciate if I could send a fixed amount that isn't necessarily zero.
As an example: "Leave 1100€ on my account as this is my monthly budget (including a security "margin"), and transfer any surplus to savings account".