Why use Joomla over Wordpress?
In the end it depends on what your after, both are very useful tools for building websites.
Wordpress is more of a blogging platform and it is built to help build simple websites easily. It is reasonably easy to make changes and build.
Both are template driven and bopth have a variety of plugins, modules and components that can be used to enhance your site.
Whereas Joomla is a framework that can be used to create super functional websites that can be customised in numerous ways.
One of the easiest ways to demonstrate this is that you can actually integrate a full working version of wordpress inside of joomla, whereas it is impossible to integrate joomla inside a wordpress site.