The benefit of a Ruby-specific JSON parser is that the parser can directly output Ruby objects. Generic C JSON parsers generally have their own data model, so instead of just parsing JSON text into Ruby objects you'd be parsing JSON text into an intermediate data structure and then walk that to generate Ruby objects. That'd necessarily use more memory, and it'd probably be slower too unless the parser is way faster.

Same applies to generating JSON: you'd have to first walk the Ruby object graph to build a yyjson JSON tree, then hand that over to yyjson.