 <p>Besides helping you getting your job done it's a job of almost every framework out there. Check what <a href="http://rubyonrails.org" rel="nofollow noreferrer">Ruby on Rails</a>, <a href="http://www.djangoproject.com" rel="nofollow noreferrer">Django</a>, <a href="http://en.wikipedia.org/wiki/Java_Platform,_Enterprise_Edition" rel="nofollow noreferrer">Java EE</a> do and you'll see exactly what I'm trying to say.</p> <p>You may find it implemented more frequently in frameworks written in dynamic languages since they allow easy extension of the language by writing DLSs.</p>