Why use a CMS, and why Joomla?
A CMS Content Management System, helps web sites that have a lot of content and those that rarely by having initiative interfaces.
This also applies to building a new website, the core coding has already been done, thus reducing development time and cost.
The web designer and the web administrator i.e. the client to be more flexible in how content is organised and managed on a site. You have a site that rarely needs to be updated or one that has be updated everyday, with a CMS you are in control of when these can be done.
Joomla is one of the most reliable and feature rich CMS available. It has many built-in SEO features that keeps up with the latest standards. Web applications can be added to it to fit specific requirements, these can third-party builds or bespoke.
Compared to Wordpress most commonly associated to being a CMS, Joomla was build from the beginning to be one, and it has been improved over the years to be the best Open Source CMS, where as Wordpress started life as Weblog and uses started to use it as a CMS by adding third party plugins.