Planning, yes, but that’s a verb that casts a very wide net.
When might one write PDDL? Be it specific tasks, or industries it is used in - the examples I’ve found online all have a robotic theme, yet the idea seems much more general.
What do they do with it once they’ve written it?
What does it solve (as opposed to just having the existence of a file that outlines objects, predicates, actions etc)?
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.