PDDL files do outline objects, predicates, actions, etc. in a machine-readable way, but in a much more expressive manner than can be done with something like JSON.

PDDL is designed to be machine readable, but also human-readable and writable. I would say you would write PDDL when you want to provide a description of the rules of a domain to an algorithm that does automated planning and acting. This could be an autonomous agent of any sort, doesn't necessarily have to be embodied/robotic in nature.