Why are documents required to change a debit date???

Can someone please advise what process to take... I called capitec to change my debit date for my overdraft as I had started a new job and my salary date had changed. They advised me that they require a letter from my HR department to confirm my salary date.

I don't understand why documents are required to change a debit date.... I tried... If the debit order is unsuccessful they mustn't hold me liable for the unpaid debit because ultimately... I tried and they are making it difficult.