Some sites redirect a visitor to the home page when a page is visited via for example a search engine. Most likely the address of the referring page is checked and when it doesn't belong to the site the visitor is forced to the home page. Many visitors will give up on trying to find the page back which they intended to visit. Also some browsers don't report the previous (referring) page correctly or even not at all. It is quite likely that visitors using such a browser can't even visit other parts of your site. Redirecting this way undermines the usefulness of search engines and bookmarks.