Often used in the subject of emails or newsgroup postings to indicate that the content is not strictly relevant. This is normally done in an attempt to avoid complaints about the message (but rarely makes any difference)
Also sometimes used when taking an off-topic observation onto an otherwise ontopic message. Eg:
> Was Bob the founding member of the Raging Chickens?
Obviously, if you'd read the FAQ you would know that Bob didn't join the band until 1976.
(OT: Usenet isn't as good as it used to be, is it?)