Versioning your API is one huge benefit and better done using proto/rpc. If you change your Json schema, have fun propagating that change to all clients without fear. Or hope you've built special infra to do that.

Also "just more efficient" is a funny way to characterize the performance difference between just data bytes vs data + structure bytes (read: the gap is large). You gain in transmission and you gain during deserialization / parsing.

Here is an example.

{"My key":"my value"} has n=21 characters. When you parse you must scan the whole 21 characters O(n), every time, just to read the thing.

If you instead store this in fixed size bytes, where you have some fixed # of bytes that tell you "my value starts at address 0x43", then you can skip to just the values you care about. You don't need brackets or quotes. And you can use other nifty tricks to compress the binary representation further for savings on the wire.