MongoDB is useful in order to store the whole structured data needed to build a given instance of a web page. You can retrieve the data for a given page, pass it to your client application which can then render it.
In such a context, MongoDB is very fast and reliable. But never forget you don't have relational information in your database. Which means if you change something in the structure of your webpage, ityou could be possible you don't have the dataunable to fill the holes in your already stored pages because you don't have the data needed to do so. More on this here: http://www.sarahmei.com/blog/2013/11/11/why-you-should-never-use-mongodb/