What's the advantage of using PO's rather than just straight vendor bills?

Depends upon the business you are in. For instance if I’m making widgets and need to buy materials (inventory) to make the widgets, I use a PO because - 1. I'm buying goods (inventory) that have a contracted price and / time frame/ quantity, 2. Need to have a Goods Receipt for inventory transactions and match pay For one offs/ things like you don’t know what the amount will be each invoice like utility bill, i just create the bill. For large purchases, a PO is a good idea. You know how much you will approximately spend with that vendor. (Eg My consulting services - I provide an estimate and once approved, customers give me a PO. Given it doesn’t include travel or expenses on it, but that’s small amount on top normally. And the bulk of my billing is covered by a PO)

Thanks for this (and the other explanation)

PO is also for managing your spending. With PO you can put an approval process (to create check and balance), keep track of what you are buying, vendor management etc. Whole procurement process would be PO>Receipt>Bill>payment, also can be Requisition >PO>Receipt>Bill>payment. With PO process you can generate 3-way matches, variance, accrual etc too.