为什么要为整个网站使用单个index.php页面？

I am taking over an existing PHP project. I noticed that the previous developer uses a one index.php page for the entire site, currently 10+ pages. This is the second project that I have seen done like this. I don't see the advantage with this approach. In fact it seems like it over complicates everything because now you can't just add a new page to the site and link to it. You also have to make sure you update the main index page with a if clause to check for that page type and then load the page. It seems if they are just trying to reuse a template it would be easier to just use includes for the header and footer and then create each new page with those files referenced.I am taking over an existing PHP project. I not