[Also a googler]:

PB is used for a wide variety of things, its a lingua-franca of data interchange within Google, but web (frontend) still often uses json. So if I want to have some proto in a database and render it on a UI to a user, somewhere in there I'm probably going to be translating the raw proto to either json or json-like (I'll admit I'm not sure how stuff like grpc-web fits in here, if you can just get proto-serialized bytes passed around in some a blob, but there are some pretty low-overhead json proto encodings that exist anyway).