It was originally designed as a game asset storage format[1], wasn’t it? Which is why the reference implementation assumes trusted input by default[2], for example, unlike Protobufs, Cap’n Proto, or SBE.

[1] https://opensource.googleblog.com/2014/06/flatbuffers-memory...

[2] https://google.github.io/flatbuffers/flatbuffers_guide_use_c...

I've primarily used it as the serialization format for choice for encoding large files written to disk. The ability to mmap it and not need to read the entire file to access specific data was very useful. My understanding is that this sort of usecase is how it's typically used.