Because it would be unfortunate if one were to accidentally forget to authorise one's mortgage payment to be paid for a few months, only to find oneself reposse

Because it would be unfortunate if one were to accidentally forget to authorise one's mortgage payment to be paid for a few months, only to find oneself repossessed. Direct debit allows a regular bill payment to be forgotten about, but with the protection that you are informed well in advance of any changes (for instance if the mortgage interest rate goes up).