Even in cases where a company prefers to rely on their website developer to manage and maintain the website it still makes sense to have the site built using a content management system rather than require any new content or updates to the site to be hard coded into the source code of the site.
Without the means to update and maintain the website via a content management system the developer will have to spend far longer adding the new content or doing the update by manually updating the source code of the site.
This will end up costing far more in terms of time and money as it will take far longer than it would to do the site updates via a content management system.
By placing the ability to update the website in the hands of the website owner the true marketing potential of the internet can be realised.