The OPC's Registered Accounts (which that configuration setting enables) enables your store's customers to create a slimmed-down account (no ship-to/bill-to addresses are recorded in the customer's account).

Unlike a guest-checkout, this customer has a unique customer-account with minimal data-gathering (good for GDPR). A customer with a registered account can do anything that one with a "normal" account can, except that they need to enter their address information each time they place an order.