I have seen freitasm's helpful post here http://www.geekzone.co.nz/forums.asp?ForumId=4&TopicId=5585. Geekzone has a wealth of information, and often a general search returns just far too much info.
Say I am having trouble with a wi-fi connection, I'd rather search for the specific issue in the wi-fi forum, rather than have to include wi-fi in the search, which then throws up a whole lot of extraneous posts in other forum that happen to mention wi-fi (not necessarily with any of my other search terms).
In particular, I prefer to search the forums before asking a question to make sure it hasn't been answered before - but if I get 20+ pages of results, to be perfectly honest I'm probably not going to skim through them all.
For example, a search on "search particular forum" turns up one relevant obviously relevant result (and to prove me wrong from the start, not in the forum I would expect it in), but then a whole lot of less relevant info too.