It was originally created because the question class was not originally off-topic. It was a contentious subject, and so we used the tag to help classify them en-masse. Generally, this kind of "tagging the kind of question" isn't all that useful on the site, but the usage of [*-rec] for this is actually a Stack Exchange practice that dates back to Super User.
I've stated back when I started deleting those questions that we should phase out that tag after we finally started to erase our library of them.
The game-rec tag is also probably going to be phased out. Remember that one of the reasons we use "*-rec" as a syntax is that it is obscure and less likely to be used. Now that we don't want the question type to come at all, the tag is meaningless as it categorizes an off-topic subject matter. We don't tag shopping-recommendations, and those are equally off-topic.
As such, please do not tag future game-rec questions as "game-rec". Please just vote to close them and direct them to the policy in our updated FAQ. The "good" questions that still have the tag will probably be swapped for some different tag.
The tag currently still persists because the tag has not been completely removed yet, so it's not unusual to imagine that some of the policing users familiar with the policy on the enforcement level did not get the memo about not adding the tag.