This is because there are a lot of content-rich databases, .pdf documents, etc. that are not indexable by regular search engines. Search engines are meant to retrieve HTML pages/sites. What if you just wanted to track a few packages but you sent one by Fedex and two by UPS and didn't want to have to deal with multiple companies' tracking Web pages? Invisible-Web.net has pulled out some of the more outstandingly useful examples of the invisible web, one being Packtrack.com?
Or maybe you will be going on vacation soon and wanted to try out a new restaurant at your destination. You could click on InvisibleWeb.com, click on Restaurants (in the hotlist on the left), then Dinesite.com or Zagat.com to find reviews for restaurants in that area.
Or maybe you are looking for the latest on your favorite celebrity. Click on Complete Planet.com, then People, then Famous People, and then Celebrities to find Our Loved Celebrities.
New Web sites & Web pages pop up everyday and more specialized databases are made available for Web searching every day. Search the hidden (or "deep") Web if you don't find enough with your regular favorite search engine and find information you couldn't access before!