Its as Skid said. With BPAY, payments get allocated to the right account automatically.
With Pay anyone companies need to rely on people entering the correct number manually, in the right format and make no typos.
If there is a typo or the number is missing it's a lot of manual work and reconciliation for them, to find where the payment belongs too.
It's also better for the customer, as they don't get payment reminders

.