Back in the day, it wasn't used for private API's either. Different teams had come up with different ways of encoding protobuf-style messages as JSON for web apps.

For the best browser-side performance, usually you want to use browser's native JSON.parse() API call and this doesn't really let you use unmodified protobufs. In particular, you can't use 64-bit ints since that's not a native JavaScript type. Meanwhile, server-side folks will use 64-bit ints routinely. So if the server-side folks decided on 64-bit integer ID's, you need workarounds like encoding them as strings on the wire.

JavaScript has BigInt now, but still doesn't natively support decoding 64-bit integers from JSON.

It didn't seem like the gRPC folks understood the needs of web developers very well.