I just learned of [Purchase Orders](https://en.wikipedia.org/wiki/Purchase_order), which sound like they are used for purchasing expensive things in B2B. For example, if you wanted to purchase a large factory machine for several $k:

I have only ever really used a credit card (and checks and such). Even if you were to pay for rent or a mortgage (paying for on the order of $100k+) house or something, you just create a contract and then do monthly payments with a check or something similar like a direct deposit or transfer or something. So you don't need "Purchase Orders" or anything like that here. Which makes it seem like Purchase Orders might be either a *legacy* thing, or if not, maybe there is some other need or benefit to using them (in which case I would like to know why you need to use them over a check / credit card).

So I'm wondering if Purchase Orders are legacy, and that they could be replaced with todays technology just with a credit card or check. Or if they are not legacy, and in fact they are very necessary, in which case I am wondering why they are necessary (and why you can't accomplish with a credit card / check what you can with a Purchase Order).