A site with a lot of content is excellent – Google really likes sites that have continually updating page content and multiple pages, all deep linked to create a formidable linking strategy. However, depending upon which platform your site is built on, when you add new pages you are sometimes replicating or even deleting old content…without the old page coming down!
One of the most damaging SEO aspects your site can have is the dreaded ‘404’ page. We’ve all seen the ‘sorry, this page does not exist’ message, and then we’ve instantly gone off that page. No harm done, right? Wrong. Your site is still being indexed for this page, and Google is noticing that the page does a) not have rich content and b) is being immediately exited by every single user. In regards to how this affects your page ranking, it can be catastrophic.
An obvious reason why a dead link will occur on sites where there are numerous different states but for the same logic. For example, if you have a team page with individual staff member pages, when you add a new member your site may regenerate the same content but you will have removed the content on a staff member that has left. But that page is still there…
Use tools like SEO Moz to find these pages and have them removed. People will struggle to find you if you don’t.