Why would you want to do that? Not questioning you on it...just wanting to know the advantages to doing that.
Okay, here is one possibility...
Surfers like to quickly scan a page before choosing what to read in depth. It is therefore a good idea to make that easy for them, so rather than having full posts on main and archive pages, show them summaries.
WordPress has an "excerpt" feature, but it only pulls out a certain number of words from the beginning of a post. More useful for the above-mentioned summaries is the optional "extract" box into which you can write a concise but informative summary of a post.
Whichever method you choose, you could decide to show, say, 10 such summaries on your main page to make it fast loading and easy to scan, but - because by selecting a category or archive the surfer has indicated a more specific interest - allow 20 summaries to display on category and archive pages.
And another...
Perhaps you let full posts show on your category and archive pages, in which case you might want to have a global setting of 5 posts or less. But good usability practise suggests that your main page would be better with at most a single post and/or a longer list of summaries. Once again the over-ride is useful.
Why would you want to do that? Not questioning you on it...just wanting to know the advantages to doing that.
query_posts is also useful if you want to exclude a category, or if you only want the front page to use a specific category, etc.. very useful. Was just learning this myself. I should've known to ask jayeff first.. and not have wasted so much time researching it myself.
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