If you have a customer base that is likely to order more than once, you may want to turn on Express Checkout. This allows customers to create an account with a username and password. The account saves their shipping information and makes checkout faster.
Express Checkout can be turned on or off at any time under 'Edit Store' -> 'Payment Processing' -> check 'Offer "Express Checkout" '.
If you sell downloadable products, Express Checkout will always be on as customers will need to login to retrieve their purchases.