Well, what if you wanted to display part of that document or information about it on another part of your site - such as it’s title or the date it was published. That is easy to do with a db-driven site such as modx, but how would you accomplish that with static html pages?
Could you easily create a dynamic menu (such as Wayfinder) that updates whenever new pages are added to the site? Could you set certain pages to be excluded from the menu?
What if you want to store information about your site users, allow people to create an account, make lists, publish their own content, comment on articles, access restricted areas...
What if you wanted parts of your page to change dynamically depending on the user input. Could you build an interactive web site such as facebook or Twitter using static html pages? Don’t think so.
How about searching your website? Sorting and filtering search results?
Sure, there are ways to do some of the above without a database. But why make things more complicated - and probably much less secure - than they need to be? With properly cached pages there is hardly any performance advantage gained from serving static pages over dynamic ones.
Just a few reasons I could think of