It's another way of showing everything that's in your site, and demonstrates why it is so important to go back to your other pages for more details. The summary page is also another honest way to get people inside to check out your stuff. It's kind of like cheating, but it's such common technique among students that most people do not escape its lure when truly desperate. It's no different on the Internet. Another reason to include a summary in your site is that it doesn't make sense not to.
Text is technically the cheapest commodity on the internet in almost every way.
It takes up almost no disk space. If your site is heavy in graphics, the summary page may be your best bet for those whose browsers can't support them. The best way to prepare for a summary page is to systematically go through each page on your site. Extract enough information to write three concise sentences about those pages. I find making lists to be the most beneficial tool when doing this.