For miscellaneous discussions and remarks not suitable for top-level posts even in the Discussion section, let alone in Main.
(Naturally, if a discussion gets too unwieldy, celebrate by turning it into a top-level post, just like in the good old days.)
I feel that way about everything. I wish there were a feed that would show all threads and not delineate between Main and Discussion.
http://lesswrong.com/r/all/ does that for posts, but it looks like http://lesswrong.com/r/all/comments just shows comments from the main section. Perhaps the latter behavior should be reported as a bug? In the meantime, if you want an RSS feed, combining two rss feeds into one is pretty trivial with yahoo pipes.