Ebay collects payment from your buyers, and then you can withdraw cleared funds, which are then paid into your bank account.
The direct debit, which you set up at the same time, could be used for refunds - if you did not have available funds - and things like listing upgrades, if you listed an item with a reserve or in two categories for example.
There may be times when you pay money back to Ebay. This is why you have to agree to the direct debit.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)