For increased security! Rather than issue you yet another username and password to remember, we are relying on Google keeping an account for you most securely, so by you authenticating with Google we trust them to tell us who you are.
Google has many account security features like verification via voice and SMS, two-factor authentication, password recovery options, even using your smartphone to login. You can have multiple Google Accounts; one for work, one for personal use, one for school, and maybe one especially for these kind of things. You can even change the email address on a Google Account later!
Pro Tip: Always register for a tournament with the same email address you use for Deck List Creator, so your registrations are automatically added to Your Registrations list.