ML can be applied in a great number of ways. One such way could be content moderation, especially detecting people who use alternate accounts to reply to their own content or manipulate votes etc.

By including IP addresses with the comments they could correlate who said what where and better learn how to detect similar posting styles despite deliberate attempts to appear to be someone else.

It’s a legitimate use case. Not sure about the legality… but I doubt google or reddit would ever acknowledge what data is included unless they believed liability was minimal. So far they haven’t acknowledged anything beyond the deal existing afaik.