When you log in, your account details, shipping addresses, payment info, equipment details, catalog of parts you've researched and your order history are all saved for you. No need to type it all again. That just makes things WAY easier.
Plus, you'll be able to access any discounts or contract pricing that may be available to you. That's important.
The real question is, why wouldn't you log in?
360-degree spin
360-degree spin