These musings on posting comments will likely be most useful to newcomers and lurkers who are gathering up their courage. I hope though that some of them may also interest the long timers, and I encourage folks to share their own strategies. Why post comments? Let me count the ways.
To share a relevant and new thought
Clearly, if you have relevant information for an ongoing discussion, share away! The more data the better. But you may have noticed the "and new" part of this reason - if several folks have already shared that relevant fact, just recommend their comments and keep reading.
To correct a factual error
This is valuable if done right. (Not done right: my friend's brother who stood up in his MIT class and said loudly, "You're wrong and I can prove it!") Doing it right usually includes a link to a useful source. It also addresses the error just for correction, without any reference whatsoever to what an incredibly ignorant dunderhead the previous commenter must be.
To make an anchor for your personal "current" list
If you look at the top right when you're signed in, you'll see a link to "COMMENTS." That link brings up a list of all your comments. I find it is a handy way to revisit diaries of current interest, because in each comment link is included a link back to the diary where you made the comment. Very handy when you can't live at Daily Kos but want to watch a rich argument (in the good sense) develop over time. I used this to check in on BoiseBlue during that worrisome evening, as another way to use this personal current list.
To reply to a reply
Part of what makes Daily Kos rewarding (and infuriating) is that not only can you share a piece of your mind, so can other people. Sometimes they share it with you specifically using the "reply" function. The link at top right will show all the replies to your comments that have been made in the past two days. What's especially handy about this is that it displays the replies in the chronological order of the replies, not the comments. This is especially valuable for kogs in widely divergent time zones or cycles. Without it bigjac and I might never get in touch with each other.
To share a personal experience and flesh out abstract arguments
Logic and policy are all well and good, but what matters in the end is the lived life. Your own particular experiences can sometimes illuminate or demolish an argument in a way that no amount of statistical data can. Please share!
To offer encouragement and sympathy
kos talks about community and building mutual support. This most often happens in the comment threads, where we tell each other good news, encourage with words of wisdom or experience, josh each other and let each other know that the lonesome trail they are following can have a better end.
To urge a comment to grow into a diary
A special subset of the previous reason happens when you read that four (or eight or ten…) paragraph comment, that is beautifully written and thoroughly documented. You feel the urge to encourage the commenter to turn it into a diary. Give in to that urge! Though the comment threads get some attention, a diary gets more. There are few purer pleasures than seeing a diary that grew out of such encouragement end up on the Recommended list.
To nurture a nascent diary
The early stages of a diary can determine whether it takes flight or fades away in the distance. Adding a comment to a recently posted diary, even if the comment is somewhat low content, can both encourage the author and give a little tick of visibility on the recent diary list.
To address a logical fallacy
Logic is hard. We all fall short at times. A gentle and respectful correction is helpful to everyone, especially if you can manage to do it without denouncing what an incredibly ignorant dunderhead the previous commenter is.
To make people laugh
Laughter is good. We need more laughter in the world.
Leap the orange croissant for some reasons in the comments -
From joedemocrat - a great one! Most diary authors know more than they wrote, so ask away! Or if something confuses you, ask a question rather than make assumptions.
Someone once said in a comment that every day you learn something, it is not counted toward your total lifespan.
Another reason I'd like to add: just to BS, converse, hang out. But this should only be done in diaries that are there for that reason, and we have several daily diaries that are more or less open threads.and from HoundDog:
to say hello to an old friend or, to inquire about a person's sig line, and life philosophy
A very deep and powerful reason for a comment, which was pointed out by noweasels, who graciously shared one of the loveliest apologies I've seen here.
To let people know that a comment has been sent to Top Comments
As Steveningen points out, Top Comments makes an easy way to share great comments. Go take a look and add firstname.lastname@example.org to your contacts list - makes it really easy to send them in!