By default, Google will attempt to spider every page you have on your site. Occasionally there are valid reasons why you do not want this to occur. Perhaps you have a slideshow presentation which needs to be viewed from start to finish. It would be awkward for someone to be dumped in the middle of the presentation. The excerpt allows users to see the context in which search terms appear on a web page, before clicking on the result.
Users are more likely to click on a search result if it has a corresponding snippet.
The cached page appears to users exactly as it looked when Google last crawled it. Don't index the current page, and do not continue following links from the page.