posted this on April 4, 2011, 15:40
Excerpts are useful when you want to display only part of your post content instead of the full post - usually used for the introduction or a summary of the post, or when the beginning of the post doesn't really say what the post will be about. (For example, maybe you start the post with a quote or a story, and then move on the the main topic.)
Excerpts are optional, and where they are displayed on your site depends on the theme you are using. For example, some themes may display the excerpt on your homepage, RSS feed, or your archives page. Not every site theme uses excerpts. You can try inserting an excerpt in the most recent post on your site and then look at the site -- the main blog page, the archives pages -- to see how it changes them.