Comment by jviotti

Exactly! The real hardcore savings will always be when you pass a schema, as JSON BinPack uses that to derive smarter encoding rules.

However, schema-less is very useful too. The idea with supporting both is that clients can start with schema-less, without bothering with schemas, and already get some space-efficiency. But once they are using JSON BinPack, they can start incrementally adding schema information on the messages they care about, etc to start squeezing out more performance.

Compare that with i.e. Protocol Buffers, which pretty much forces you to use schemas from the beginning and it can be a bit of a barrier for some projects, mainly at the beginning.