A CMS allows you to login to your website and change your own content without needing complex skills or knowledge of the underlying technology. This saves you having to hire someone in order to change content on your site. You can change it whenever you want, from any device, which will make you feel awesome! I use a CMS called Processwire as I have found this to be the most user friendly system to date and having a very active community means code is updated frequently and issues are quickly solved. For the record, I have previously used (and enjoyed) Umbraco CMS, Wordpress CMS, Drupal 7 CMS and Craft CMS but after trying Processwire for the first time, I was hooked.
One of the best features of Processwire is that it works well on mobiles and tablets. The admin area features a great responsive design which makes editing your content very simple indeed. You can access your content from wherever you are by simply logging in to your website. Just login, edit, click 'Save' and that's it, your changes are immediate and you can view the new content straight away!