If you are a web programmer that designs website for clients then your client may require access to the back end system of the site to allow them to upload additional content, change or add products etc. Some programmers use an off the shelf content management system such as WordPress or Drupal and others use their own. If you are using your own content management system then you need to carefully consider how user friendly it is. As a programmer, you will be familiar with HTML etc., but there is a good chance that your client won’t be. Once you have built the back end system, why not get a friend (who has no developer experience) to log in and see if they find it easy to do the tasks that your customers will be required to do. IF they struggle, then you may need to rethink the process.
If you are using an off the shelf system, then these are usually developed in a fairly straight forward way, but if you have made changes to the templates or themes etc. then you may need to check everything still works as it should. You may need to install extra plugins to get the pages and content to work how you need them to.