Actually I think they can still add items to their cart and pay at once. They might not get the shipping discount though, if that is something you'll need to invoice for. I've removed the IPR from listings if the need arises.Adding to Catbird's reply,
To add immediate payment required to all your BIN listings you can use the bulk editor.
Go to your selling page & tick the box next to each item you want to update,
At the top select "edit" and then select the "payment" section.
Select add IPR to ALL and go through the save processes.
The only benefit I know is when you sell lower priced items; buyers can add several to their cart & pay all at one time and/or if you offer a shipping discount for multiple purchases.
I think that if the buyer doesn't have an ebay account, like one of those newbies where ebay creates the account on the fly, has to pay immediately even if you don't have the box checked, because the account doesn't get created until they purchase and pay.