The signature refresh notice says that i can also provide my date of birth if i wish, but this is not compulsory. why is this?

You are not legally required to provide your date of birth. However, it would, be helpful if you did. If you do wish to provide your date of birth again, it will confirm that we hold the correct date on our records. If you provide us with a different date of birth from the one we already hold on our system, we will amend the date you gave us previously.