Joomla is widely used as an open source CMS. It is created specifically for users who want to build websites and at the same time require various other powerful modules.
Joomla has an active and powerful community. Because of its open source and large community and variety of plug-in availability, it is cost effective.
It is simple to manage own website and content.
Joomla is used to create websites from CMS, blogs to e-Commerce applications. Joomla is also used to create a custom web application or software needs as it is more powerful than other CMS.
Joomla has VirtueMart, which is used as a great tool for e-Commerce. It also has a good amount of stable extensions which can be plug-n-play. So no extra development time or cost applies unless you require any custom thing which no one has tried before.