Content Management Systems nowadays are built in way that everybody just familiar with any form of user interface and a little experience in the IT world can perform most of needed administrative tasks such as plugin deployment, translations, changes to the graphical layout, etc.
The problem comes when such admin user crashes into a malfunction in certain plugin or incompatibility between plugins, or some server related error. In these ( and thousands of others issues ) a Web Developer with experience may come in handy.
For the web developer it is crucial to be familiar the anatomy of the system ( from its core to the most used plugins ) order to be able to investigate, locate and eradicate the problem.
Being able to solve a case in a timely manner can be difficult task if the web developer have not been in a similar situation, in my experience most systems that have any medium level of customization are a complex organism that works despite the technology flaws. So any change in this fragile ecosystem can lead to problems and technical flows.
This is the reason that having a true web developer in hand can be a time and money saving necessity.