The reason Odoo requires this field is because Odoo does not differentiate a contact as a Customer or a Vendor. They're all contacts and live in the same contacts module and the only difference between the two is whether or not you choose to use that contact as a vendor on a Purchase Order/Bill or a customer on a Sales Order/Invoice.

The moment you do so Odoo flags them as a Customer or a Vendor but in reality a Vendor could become a Customer and vise versa.

Odoo probably views this as the accounts payable of the customer (not your company's) and the accounts receivable of the supplier (again not your company's).

This way it makes sense as customers can only have an amount that they still owe your company (which will be seen as accounts receivable for you and accounts payable from the customer's perspective). Vice versa for the supplier/vendor.

Hope this helps!