Comments are often deleted when they're obsolete. And whole pages of comments are often removed by moderators if they're judged not to contribute (or not to contribute anymore) to the question.
The criteria for deleting comments here on meta is more stringent than on main. Here on meta, a comment must usually be clearly obsolete or clearly not constructive or rude to be removed. That is because discussion is valued on meta far more than on main, where everything is subordinated to the goal of asking good questions and creating good answers.
However, it's not true that as the OP you should never comment. If someone has commented to request information, and you have good reason to think the information they're asking for is irrelevant, then you might provide it in a comment and ask them how it's relevant. If if turns out not to be relevant, then it never belonged in your question in the first place. (And you can delete the comment!)
Furthermore, sometimes we comment to notify someone of a change. Commenting on someone's post notifies them, and if someone has commented on a post, your comment notifies them if it contains
@theirname in it. (You can only have one of those in a comment, however.)
You might provide information in a comment because, while you think it's valuable, it's intended to be transient. There are two general cases of this. If someone asks something that's not related to your question, but just to help them with some other thing, then there's no reason for that to go in your question. There are plenty of secondary queries posed in comments (especially on answers) that are quite appropriately answered in comments and not by editing the post. (Though often they might be even better as separate questions.)
The other general case is if you cannot get the information into your post in an appropriate fashion. For example, a user with less than 10 reputation cannot include more than 2 links in any post, on Ask Ubuntu. They also cannot post images. So they might provide a link, or a link to an image they've put up elsewhere (like http://imgur.com/ or http://postimage.org/) so that a user with higher reputation can add the link or image to the question. This is a perfectly fine use of comments; once it's been added to the question, the comments can be removed. (Ideally, by the user who posted them--but there is certainly nothing wrong with flagging them, or any other comment, as obsolete once it is).
So, should information important to your question be in your question itself? Yes.
But to say "Please provide additional information only by editing your question and not within a comment" (emphasis mine) is an overgeneralization. In my opinion, that canned comment creates confusion, and a less broad statement would be more appropriate. Something like:
The best way to add additional information to your question is by editing it, with the edit button. It is best seen that way, and comments are primarily for secondary, temporary purposes. Comments are removed under a variety of circumstances. Anything important to your question should be in the question itself.
I'm not saying anyone needs to adopt those precise words. But most claims of the form "never do X" "use only this method" are false some of the time.
I usually avoid the problem in the first place by explicitly requesting that information be provided by an edit, when I ask for information. Once information is provided in a comment that ought to be in the question, I edit the question to include it (unless I'm unable to understand it), then flag the comment as obsolete. It's rarely necessary to post a comment to tell someone not to use comments.