As The Verge discovered, Verizon is offering a similar $400 discount, but it's spread over 24 months and chains you to a contract. By contrast, the Best Buy deal gives you the $400 dollar savings immediately, with the balance of the phone's price going on your Verizon account. That means you could pay it off over time or immediately, then switch to another carrier if you choose, presumably while making a symbolic "dusting off hands" gesture (Verizon is Engadget's parent company).
You'll still need to pay the $30 activation fee, however. If you just want an unlocked Pixel 3/3 XL phone free and clear, Google is offering the Pixel 3 for $150 off and the Pixel 3XL at a $200 discount. All of those deals prove that it never pays to be an early adopter -- especially for phones released in the fall.