Why do I need to provide information on income and expenditure to agree a payment plan?

Barney says:

As a vehicle finance provider, we have a responsibility to ensure any payment arrangements we agree with customers are affordable and sustainable; if we ask you to complete our income and expenditure form and give us full details around your income and outgoings, it’s so that we can fully understand your other financial commitments and make sure that any payment plan we agree with you is set at a suitable level.