Every user can post their own musings in their own section. Basically, it's as if every user has his own personal weblog on your site. The Diaries section can be used for anything the users want to use it for: getting to know each other, venting, or requesting feedback on a draft of an article they're working on.
If the variable use_diaries is turned on, diaries are available to all users with the perm story_post (A.12.35). Diaries are not shown on the ``Everything'' index page, nor are they included in the site's RDF feed (see section 4.5 for RDF information). Diaries are not sent through the queue. Apart from that, diaries have all the same features as stories and are edited in the same way.