...I can call up now and use my mums phone number (instead of account number) and address and DOB no problem but I will not know her security question answer.
I think you meant password, but otherwise you're exactly right.
A password is a quick way to prove that you are who you say you are to the person you're calling.
Your security questions help establish your identity enough to be able to reset your password if you forget. For anything longer term, they are relatively insecure because they do not change. You can have different passwords for each of Virgin, your bank, and the electricity company; but your mother's maiden name will be the same for all three.