A CMS is used generally by the website owner, to manage the content on your own. It's a web-based system that allows you to build your website without needing to be a programmer.
With a CMS, you can add unlimited pages to your website, modify and sort navigation menus and widgets, manage categories and news articles, blogs, control content such as homepage sliders, and much more. Basically, anything you wish to control you can do on your own with a CMS.
Do you need a content management system?
You don't necessarily need a CMS, but one could make your life easier. Unless you're a programmer on your own, without a CMS you will need to contact your web designer whenever you want to make a change to your website. Some of our customers are fine with that arrangement as they are not technical and would rather us manage the content for them. Some other clients are interested in managing the content without having to contact us. It all depends on how large the website is, and the needs of the client.
Are Content Management Systems more expensive?
Generally they carry an additional cost up-front as your website will have to be "integrated" into the CMS backend. But over the long run, you could wind up saving money by managing the content on your own without having to rely on a web design company to do it for you.
Additionally, a CMS will need to be built with a programming language and a database to store all of your data that powers your website.