The are three basic use cases:
Buyers pay with PayPal accounts: If the buyer has ever logged in to a PayPal account using the browser, the cookie left by PayPal causes the second of the two checkout pages to open – the log-in only page. It is optimized for people who already have PayPal accounts and expect to see just the PayPal login box. You will get a screen with a small link to the Credit Card Option. Although this is not as obvious to the customer, the Credit Card Option is still available. For example, the cookie that holds the cart info will retain that info for 21 days. The only way it will change is if you remove the items selected from the cart or delete the cookie.
Buyers pay with credit cards: On the other hand, if the buyer has never logged in to a PayPal account using the browser, PayPal opens the first of the two initial checkout pages, where buyers directly enter their billing information in the left-hand form.
Buyers using a public terminal: On a public terminal, PayPal uses the cookie left by the last buyer who used the terminal. If the last buyer paid by PayPal account, the next buyer will see the log-in only page. If this next buyer also wants to pay with a PayPal account, the optimal page is presented. If the next buyer wants to pay by credit card instead, the log-in only page provides an “escape hatch.” Buyers click a link on the left-hand side to open the page where they can directly enter their billing information.