For your convenience
Creating a store account is entirely optional. If you prefer, you can purchase items as a guest without setting up an account.
So, why bother setting one up? Once you set up an an account, with just your email and password entered, for every future purchase, your account information will be automatically stored. You will not need to fill in the billing or shipping information unless there's a change at your end.
As for the safety of your credit card information, we do not have direct access to that information beyond the last 4 digits of the card that's used.
By establishing an account, you also have access to your past purchases (unless you made a purchase as a guest) for future accounting purposes.