I'm noticing that my comments are getting deleted, and I'm wondering what the feel is here for that. According to this, it seems as though the purpose for comments is for asking follow-up questions and not "comments" per se? Perhaps the one common element to my comments are that they are 'comments' and not 'questions'. Am I understanding that correctly?
Over on Security.SE, comments and even min-debates are valid, but I know and respect that every community is different.
Comments are for a brief request for clarification or improvement, and they can be deleted after the querent or respondent performs appropriate edits. If sufficiently sure of yourself, a witty Bon Mot can be added, though it's likely to be swept up in general purges if not highly upvoted.
Comments can be unhealthy so they tend to get cleaned up quickly — arguments are very easy to have about role-playing games. Responding to a comment is almost always not worth it, as it takes effort that can otherwise be used to improve a votable resource.
Especially unhealthy usage of comments includes: starting an argument, leaving a comment on an answer only to tell them they're wrong and/or their answer's bad whilst suggesting no means of improvement (just downvote, and if you can, provide a better answer), or thanking someone for their help (just upvote), or asking them about entirely unrelated problems in comments (this is help vampirism; you should start a new question instead so people can use the proper answer mechanisms to respond).
Invite people to chat to have debates, so that only people who want to voluntarily subject themselves to the argument can see. (You can use [chat] in comments, make your own room, or link to the main rpg chat room.)
The intention here is to focus on improving question and answer quality. Conversation is for chat or a forum. And unfortunately, comment policing isn't something that can be done halfway, as hanging comments tend to attract arguments and discussion and thereby decrease quality.
It's also worth noting that much of our deletion prompts come from commendable user flagging of comments that have become too chatty, argumentative, or correctly have been edited into their respective posts.
In case Brian's answer isn't quite clear:
Comments are for providing feedback on questions which allows questioners or answerers to improve their question or answer. This can take the form of suggestions for improvement, requests for clarification, or constructive criticism (such as pointing out a mistake or error, not just saying "this is bad"). They'll generally get deleted sometime after the clarification or improvement is made. If a mod is active, and they have their eye on the question or users are flagging comments for cleanup, that could be very quick.
If your comments are comments which don't provide anything that can be used to improve what you commented on, they are likely to be purged by a mod. Note that even if they do provide this kind of information, if the poster edits their post to take your comment into account, your comment is also likely to be purged. (It's served its purpose by helping the poster improve, so it doesn't need to be there any more.)
Note for clarity: This all applies to the main site. On meta, in many cases it's almost the opposite. Discussion in comments is perfectly legitimate on meta and rarely, if ever, gets deleted.
Something to note, although the mods may not agree: a comment can be useful even if it's soon deleted. I find it perfectly appropriate to make a comment that I know will be cleaned up sometime if it serves a greater social purpose.
For example, I just had a comment exchange that went like:
"You beat me to the answer! I was surprised there was an answer in the book for this."
"Me too! I had the 'up to the GM' answer all written out before I double-checked!"
This exchange enhances comradery among potential answerers and turns what could have been a grumpy "sniped" feeling into a little moment of shared wonder. Should/can those comments be deleted at some point? Sure. But I think it's worth the small amount of moderator effort required in exchange for a better community.
