(Tedious disclaimer: my opinion only, not speaking for anybody else. I'm an SRE at Google.)

Performance. gRPC is basically the most recent version of stubby, and at the kind of scale we use stubby, it achieves shockingly good rpc performance - call latency is orders of magnitude better than any form of http-rpc. This transforms the way you build applications, because you stop caring about the costs of rpcs, and start wanting to split your application into pieces separated by rpc boundaries so that you can run lots of copies of each piece.

I cannot sufficiently explain how critical this is to the way we build applications that scale.