One of the reasons you may want to do this is because there is already a site in place which drupal is being added to, and this site content does not need to be rework.
Another is because you want to add non drupal content to a drupal powered site for whatever reason. Maybe because of the shopping cart application. Or to build a web 2.0 around existing content. or a CRM system.
Another is because you want the home page to be a static page.
The reason really does not matter, it can be done and done without a great deal of reprogramming, actually no reprogramming at all.
On a standard LAMP web host the .htaccess sets what file will be called as the default page. After an installation of drupal .htaccess will have this configuration line in the .htaccess file.
Change it to.
DirectoryIndex index.html index.php