"A web content management system (WCMS) is a software system that provides website authoring, collaboration, and administration tools designed to allow users with little knowledge of web programming languages or markup languages to create and manage website content with relative ease." -Wikipedia
A CMS is a maintenance tool for non-technical people that allows for website administration through a browser-based interface. This allows users to make changes to a website with little training.
A CMS gives you the freedom and the power to build your brand, engage your potential customers and increase market opportunities for a fraction of the cost of hiring a web development team.
In no particular order, here are some reasons you might want to consider.