The encoding for protobufs is significantly more compact than JSON. If you're logging or persistently storing your data on the server, this can cut down your storage and bandwidth costs significantly, particularly if you're operating at Google scale.

Haberman also mentioned the schema benefits.

All that said, I'm using JSON for my current startup. I view them as optimizing for different parts of the product's lifecycle: JSON lets you quickly adapt the protocol and switch out different languages for different services when you're figuring out what product to build, while Protobuf saves you money when you're trying to scale it. I'm also pretty intrigued by Cap'n Proto as a high-performance serialization format, since it fixes a lot of the problems we faced using protobufs at scale at Google, but its language support just isn't up to protobuf/JSON yet, and the protocol is quite complicated.