The reason parent_comment_id matters more than it looks: a Threads conversation is a tree, and a flat CSV normally destroys that. Because every reply carries the id of the comment it answers, you can pivot the file back into threads, count how many replies each commenter drew, or filter to first-level responses only — without touching the site again.
Comment rows are flattened for you on export. Underneath, a run stores one record per post with the whole reply tree attached, so re-exporting the same session later gives you the same rows in the same order.
The Threads Comment Scraper covers comment exports in full — including how nested replies and hidden replies are captured, and what a single busy post yields with each switch on or off.