When I review answers in the low-quality queue and choose Delete or Recommend Deletion, respectively, I can choose from a series of comments or No comment needed. What effect do they have and when should I choose such a comment?
TL;DR: The canned comments are really just what you see; there is no hidden function. Use them only when they really apply and provide useful guidance.
What does it do?
If you choose a comment from that queue, it will be left as a regular comment on the answer with you as the author. The system will add a link From Review leading to the respective review item. If somebody chose this canned comment before you, their comment will be upvoted on your behalf instead.
And that is it. Your review or the post won’t be treated any differently because you chose a certain comment or no comment. In particular choosing This is commentary on another post does not initiate any systematic steps towards the answer being converted to a comment. There is no statistics (available to users or moderators) how often you or anybody else chose a certain comment.
So these comments only have two effects:
- They provide guidance to the author as to what is wrong with their post, how they can fix it, or what they can do instead of answering.
- They tell other reviewers and visitors what you think is wrong with the post.
How should I use them?
If you or somebody else already left a comment explaining what is wrong with the answer, upvote that comment and choose No comment needed. (If the existing comment is a canned comment from the queue, choosing the same comment again would have the same effect, however, the first variant is safer since sometimes what looks like a canned comment from the queue was posted manually or by a userscript.)
If one of the canned comments applies perfectly to the post, choose this.
If one of the canned comments applies, but there is more to say, choose the comment, go to the post, and either edit it or leave another comment.
A typical case would be if somebody posts a question as an answer that would be closed in its current state: By telling the user directly what they need to improve before posting it as a question, you avoid a tedious and disappointing back-and-forth later.
Most importantly, if none of the comments apply, choose No comment needed. Preferably leave a regular comment to explain what you think is wrong with the post. However, if you do not want to do this for some reason, it is better to leave no guidance than bad guidance.
In particular, do not use This is commentary on another post, not an answer¹ unless the post really would be a valid comment, i.e., a request for clarification, criticism on a post, a relevant link, a really good joke, or similar. Answers to another question, rants, or similar are not valid comments. Also, a long post (say, more than 600 characters) is very unlikely to be a valid comment. Leaving said canned comment on such posts is confusing for the author and does not help them to understand what is wrong.
¹ Which is (for users below 50 reputation):
This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker.