Most people have visited sites that use an introduction page. You are forced to view a lengthy animation before you can see the actual site. Most dialup visitors scramble to get away from these sites as quickly as possible because it takes too much time to download before they can even see a site they know nothing about.
We don't know why the owners of these sites want these pages, but it is not because they are trying to attract visitors. Most people shun intro pages like the plague. Surveys show that visitors, especially dialup customers, perceive these pages as a "go away" sign.
We recommend against using introduction pages unless you want your site to appeal only to visitors who are already highly motivated to use your site prior to their first visit.