The question may be, "why wouldn't you use a Content Management System (CMS) for your website?" Years ago when CMSs first became popular, they were expensive, proprietary and costly to get setup.  With the help of many committed developers and open source projects like Drupal, that isn't the case anymore.  If you're shopping for a new website or website redesign you should question anyone who doesn't offer you some variety of CMS for your website.  Some designers don't offer a CMS due to inexperience or not really understanding their client's needs.  Some more nefarious companies purposefully tie you to them and an hourly charge for any changes to your website, they don't want you to be able to make any changes on your own.