You should always create the PO if you bought something.
A PO is your record of expense, your record of spend - We need to make sure Ivy not only knows that we sold items, but also that we bought them, right? So we record the income through the invoice that we send out clients to pay, but if we do not create the PO we do not record the spend.
Even if we buy something online or we closed the purchase with our vendor over the phone, and we will never actually use the PO, we should always be creating it.
In fact, I recommend that you use the attachments (assets) on the PO to upload a copy of the receipt or invoice from your vendor. This way you can easily double check that the value on the PO is exactly the amount that you actually paid.