I am not going to reinvent the wheel by trying to create my own definition of CMS and my own list of reasons why you might want to use one. So many have already done this.
Below are several links that provide great insight into what a CMS is why you might want to use one.
- Why CMS: http://www.fivecube.com/company/inside_fivecube/approach/cms/
- Knowing When You Need a CMS: http://www.techsoup.org/learningcenter/webbuilding/archives/page9378.cfm
- So, what is a CMS?: http://www.steptwo.com.au/papers/kmc_what/index.html
- Content Management System: http://en.wikipedia.org/wiki/Content_management_system
- Why Use a Content Management System?: http://www.alledia.com/blog/magazine-articles/why-use-a-content-management-system/