There can be many reasons for this from SEO point of view. One of them can be to send Link Juice to the pages they want to rank better (May be because those pag

There can be many reasons for this from SEO point of view. One of them can be to send Link Juice to the pages they want to rank better (May be because those pages bring more advertising revenue or being new don't have enough backlinks.) So let's say you have old pages which have good amount of backlinks but the stale content is not bringing enough visitors to your site. So you can delete those pages and permanently redirect them to new pages. This way the new page will get a Link Juice boost and it will perform better in search results.