At the core the Low Quality Review Queue exists to do 2 things:
- Close low quality questions
- Delete crap (questions and/or answers)
However, despite these goals, the minimum reputation to access the queue is still less than the minimum reputation required to accomplish those actions.
But why is this? We don't trust those users enough to close questions or delete posts, but we trust them enough to perform actions that will remove the posts from that queue.
Near as I can tell, when a user without close privileges reviews a question, they have 3 actionable options, "Looks OK", "Edit", or "Close". Both "Looks OK" and "Edit" will help kick the post out of the queue. But "Close" only effectively counteracts as "Looks OK" vote and helps keep it in the queue. In essence, those users can only help remove the post from the queue and can't really do anything else.
I can say from experience that I have seen posts that I have flagged as "Low quality" get kicked out of the review queue because someone decided to perform a meaningless edit that made absolutely no difference in the quality of the post. It was then necessary to involve a moderator to handle the post. But if low rep users were not permitted to review in that queue, it likely would have been deleted without bothering a moderator.
If we can't trust users with <3K rep to close questions, what is the benefit to allowing them access to the Low Quality posts? As an extension of this, if a user has <10K and can't delete, what is the benefit of them being allowed to access the same queue to review low quality answers?
When the only action someone can do is to remove the item (or leave the post for others), why is it necessary to allow them to review to begin with?