According to the description page, the Interactive Compendium will also cause your courier to evolve. So, the more you watch and take part of the event through the Compendium, the more your courier will "reveal new features." It should be noted that purchasing the Compendium also includes a free Smeevil courier. Sounds mysterious!
So there's a lot of cool little interactive things you can do while the tournament is underway. Also, 25% of the money spent on Compendium purchases will go towards the $1.6 million prize pool. So yeah, $1.6 million was nothing to sneeze at already but it's only going to grow larger with each purchase made of the Interactive Compendium.
If you'd like to purchase the Compendium for yourself or a friend, head on over to the official Dota 2 store and add it to your cart.