Huh, I was just in some code that uses this library a few days ago. I wish it included a base64 implementation for more efficiently encoding arrays of bytes or floats. JSON is inefficient for large arrays of numbers.
Why wouldn't you use a base64 library ? This is prime library bloat mindset. Base64 has nothing to do in a JSON lib; there are a ton of implementations out there.
A base64 implementation is very small, but not completely trivial to the point where you should implement it yourself, or paste in a one liner from Stack Overflow. It's widely used with JSON. C++ has no package manager to make it easy to import tiny libraries for purposes like this, so C++ libraries should be a little more self contained than npm packages or rust crates. It would absolutely be reasonable to include a base64 implementation in a C++ JSON library for those reasons.
Sure, if you want to change the format of the whole message. If you're communicating with something that accepts JSON and you can't or don't want to change that, then having a more efficient option is nice.