Each Compendium costs $9.99, and each Compendium purchased adds $2.50 to The International 2014's prize pool for the competing teams while simultaneously contributing to various stretch goals for the game. Having reached the $2.9 million goal, Dota 2 will receive an All Random Deathmatch mode available to all players. At $5 million the game will receive a 1v1, mid-lane-only matchmaking option, also available to all players. There are plenty others as well, and the highest stretch goal, $6 million, will allow Compendium owners to customize a building in their team's base.
Valve's 2013 prize pool totaled $2,874,381 - this year, it took the community less than a day to surpass that number. You may also recall that it took less than an hour for tickets to sell out. The game is kind of popular.