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If you leave the whole article in the 'scoop' field it will appear on the main news page complete. If you transfer the article to the 'extended text' field, you can use the 'scoop' field to write a summary of the article. The scoop text will appear on the news page with a 'read more...' link, which leads to the article (the extended text).
I think the principal is that staff authors (who you would for example put in a 'Staff' user group) would have permissions to approve their own articles and would also know how to write a decent 'scoop' summary, so both fields are available.
Other registered users are required to submit articles in the one field because in that case the webmaster would normally edit the article before publication. It is then the webmaster's decision whether to leave it all in the scoop field, or split it.
By default, the scoop text also appears at the head of the article, however, you'll find a small hack
in this thread that hides it in the article itself.
A thread is for life. Not just for Christmas.