''The graphical user interface is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, instead of text-based user interfaces, typed command labels or text navigation. GUIs were introduced in reaction to the perceived steep learning curve of command-line interfaces which require commands to be typed on a computer keyboard.'' From Wikipedia.
Being able to develop graphical user interfaces will allow you to create applications that are easier to use for your every day user and thus, your work will be able to spread to a greater number of user in lesser time having a greater impact.
If you want to reach more users, specialy those that are not quite familiar with programming, you definetly need to have some knowledge in graphical user interface developing, for it is the only way in which the great majority of user out there has ever interacted with a piece of software ever.
Matlab's app designer is built with the intention of making GUI developing easy. It's graphical approach to drag and position components, alogside the way by which it walks us through the callback functions makes it the best option out there for programming enthusiast, engineers and scientists out there that need their applications to be user by more people than just themselves.
This course is meant for intermediate Matlab programmer that would like to take their knowledge to the next level and be able to reach users not accustomed to text based programms.