Creating a store account (in the pop-up window) is optional. If you prefer, you can purchase items as a guest on the checkout page without the need to login.
If you choose to set up a customer account, after you provide your email and password information on the Sign-In form, you can store your shipping and billing addresses along with your credit card number (you can choose not to store that information) when you make your first purchase. After that, future purchases will be quicker to perform.
By establishing a customer account for your purchases, you will also have access to your past transaction history for accounting purposes.