I often delete my own comments when there are typos, when they might be interpreted in the wrong way, or their wording needs improvement. I also do my utmost to delete comments that are no longer needed, so I am not a "comment whore" (I think "whore" is an expression popular in the US for people who crave attention at all costs. If someone is offended by the term, apologies, a more effective expression does not spring to mind.) Yet, on several occasions on Meta, I find perfectly harmless comments of mine are deleted when I return to visit a page.
A now-deleted comment of mine on this post attracted a couple of upvotes, I didn't keep count, yet it was deleted for it being "chatty". It was not the only victim of the culling.
The comment which was deleted was, I felt, useful, it was not idle chatter. It was useful to point out that the incriminating post was invisible for users like myself who do not have 10K on Stack Overflow, it was complimenting the OP on the amount of care and attention they spent in composing the question and at the same time reminding users that a post so well thought out and argued takes considerable time. Then there was something else, but I've completely forgotten what it was, too bad, I suppose. If ever there was a more conciliatory comment then that was it.
On a closely related issue, anything can be interpreted in the wrong light by one or more users, this inevitably leads to prejudicial treatment, and that risk increases when the person commenting is a non-native speaker. A number of non-native users' posts and comments are weak on nuance, tone and ‘diplomacy’, and some languages adopt a more direct, perhaps brusque, approach than the pleasantries the English language employs. This missing tone can make or break a post or a comment.
Here is one such question which I recently edited because the downvotes seemed to be a knee-jerk reaction to the OP's original title. After the not insignificant edit, the upvotes started to arrive.
Mods and CMs alike should always be fair, even towards users they may personally dislike or find particularly annoying, and recent events have led me to believe that I fit into that category. I have been warned of misusing comments and the veiled threat of suspension has been used as a stick to dissuade me from actively participating. This sounds like sour grapes, doesn't it? It's not. My reaction on finding that a comment (which did not violate CoC and was not off-topic) of mine was yet again deleted is of stunned disbelief.
I thought comments on Meta were viewed differently from those on the main site. I found the following topic, posted in 2015 by a highly respected mod on a number of SE sites and very recently also appointed as a mod on Meta.
[Emphasis in bold mine]
We all know what comments are for -- and aren't, but that guidance is, naturally, geared toward main sites, where we want to keep things clean and focused. Meta, on the other hand, is a site's "second place", with some different rules and conventions. For example, questions on main sites can be closed as primarily-opinion-based, but meta is all about opinions. There's a "discussion" tag and such questions collect many opinions. Meta is different.
The OP of Stack Overflow has engaged in ideological censorship and unwelcoming behavior also expressed perplexity on the deletion of several comments beneath their post. The mod who undertook this action, replied [Emphasis in bold mine]
Anything that provided context, potential clarifications/requests that didn't feel like they'd fit into an answer that was on topic but were useful or talked about possible reasons to close (since they are automatic)- and a few more that felt like they wouldn't be answers. I could have purged them all, and it was tempting, but if you're going to argue out points in comments, that's not what they are there for. And now, I'll be doing a second round of cleanups
Later, they added this further explanation
So, We'll be rather agressively pruning the comments here. Its not social commentary, just housekeeping - but feel free to post any substancial information that's of interest on this topic as a post.
I 100% disagree, it is still social commentary. Only the comments agreeing with the deletion of the answer were allowed to stand. How is that not social commentary?
It's also worth noting that it is impossible to post an answer on a question that has been placed on hold, so that comment was poorly thought out. It happens.
Henceforth–lovely word–I shall be taking screenshots of every comment I post on MSE, especially those which express disagreement with a decision undertaken by the team and do not violate the CoC. The two can be achieved.
Overall, it seems to me that the arbitrary deletion of comments follows no fixed criteria.