Usually it’s if you want to transfer money into PayPal they process it as a direct debit for some reason.
I imagine it can also be used for paying back PayPal credit that you’ve used.
PayPal normally take the fee of an item you sell directly out of the list price.
Pay Pal usually take payments via direct debit but you can also set it to do card payments insteads, I personally prefer the Direct Debit route as it gives you an extra few days and the DD protection.