BSON is more like traditional JSON. All collection elements are ended with specific sub element (just like ']' '}' in JSON), you don't need the complete document before output the first byte. So it is streamable and memory efficient when output.

But this also affect decoding. Not prefixed with size means the memory size you need to allocate is unknown at the time of decoding. You may need to allocate more than required amount of memory or grow on demand if you don't know the size at the time that decoding starts.

Beside this, BSON is also an exchange format. There is full spec and you can use it outside of mongodb. (For example, you need to write an object that contains ArrayBuffer to disk? Just serialize it as BSON and write to disk.) While jsonb in progress is just a DB internal representation, you don't actually see the binary format anywhere.