A content management system in theory allows non-technical users to edit the content on their websites, without changing the structure, navigation, programming, etc.
I say "in theory" because you can always do a better job of editing if you know some html (hypertext markup language) and css (cascading style sheets). Content managements systems all have built-in editors that have a user interface similar to Microsoft Word. But it's been my experience with any editor I've used that about ten percent of the time, I can't get the webpage to look just right unless I click on that "Edit HTML" button and edit the raw html code.
Having said that, do I believe that content managment systems still have value?
They sure do! From both the web designer's standpoint, and the client, the benefits far outweigh the drawbacks.
From the web designer's perspective, there is great efficiency in using a framework and plug-ins. It is usually far less work to fit together these pieces of a puzzle than to write all the code yourself, from scratch. You can get the job done more quickly, and therefore do more projects per month than if you wrote all the code yourself.
From the client's view, using a content management system makes a lot of sense because it allows you to make many updates to your site yourself. You don't have to pay a web designer or programmer if all you want to do is update the hours that your store is open. Yes, there will be times that you just can't get the site to look like what you want without external help, but about 90% of the time, you will - a huge savings.
One tip for website clients - make sure your designer does some tailoring of your template (for example, changing the colors and images to match your logo and type of business - this will help reduce the "cookie-cutter" appearance that websites made from templates can have if you're not careful.