A bridge page, also known as a doorway page, is a web page created for the main purpose of sending visitors to a different site. Bridge pages offer little or no unique content and are considered low value to users.
Bridge pages can also be sites that drive traffic from Adwords-compliant content to non-compliant content.
as the name suggest they act as the bridge between the website and the visitors, so that visitors can get to the site , more visitors means more traffic more traffic means more PR
Well, the web pages that are created to rank high in search engines for particular phrases with purpose to seduce you to watch another page are known as bridge pages