Long story short, Google knows how to run a pretty big network, given that it's running both its own branded services, Android and YouTube. By sharing its know-how, it's expected that the networks can cut costs on hardware and be faster to implement new features. Google uses innovations like YouTube's Smart Offline, that trickle-downloads clips overnight when data costs drop, as one such example.
It's also another step towards becoming an infrastructure provider for a much bigger chunk of the world's services. After all, Google already runs its own fiber business, as well as a burgeoning mobile network in the form of Project Fi. The search engine is also contributing to an open-source project to improve cloud computing, CORD. That puts it alongside other heavy-hitters like AT&T, China Unicom, NTT and Verizon.