As per my limited knowledge CMS in general helps you easily manage a website and allowing easier modification of content. Usually a CMS has a front end / backend system where in the front end (or design / template) is coded to output the content stored in a database. The backend system wherein any authorized user can go in and update the content which in turns stores data in the database and is finally used by the frontend to display it in a pre-described format.
Why should one use CMS?
Well to better organize the content and for easier modifications to design and content. If you recall long back when sites were made and if you had like 100 html pages, you would need to go into each one of them modify the design / layout etc. But with a CMS thats easy. Just go into that specific template and update the page layout and it would apply the same to all 100 pages as the content is generated almost at runtime.
Wordpress is simply a type of CMS. There are many such CMS available and based on each ones needs one can choose a CMS.