I guess it's to reduce the validation workload at their end. They'd rather receive numbers generated and validated by a piece of software, rather than numbers that may or may not make sense.
It might help slightly against accidental typos in the 7 numbers. But I really doubt that the transferring of 7 numbers from one bit of software to another once per year is a substantial contributor to accounting errors, when you compare that to hundreds or thousands of invoices/bills that need to be entered into the accountancy software.