They are guaranteeing for that producer their power is purchased at x$/whatever for 20 years. Massively lowers the risk profile for the wind farm, and lets them focus on potentially expanding. Google is basically acting as a subsidizer of renewable energy here. So while yes they rely on the grid, they're a market force for increasing the percent of the power produced on the grid to be renewable.

Every MW produced by the renewable is a MW not needed to be produced by natural gas/coal