I remember the first time I saw a senior engineer struggle with the command line. He was a new hire and was getting up to speed on a new project. If memory serves, he was trying to get his IDE set up for a java project.
I noticed him with the command line open, trying to remember how to move around directories, and wondered why he was having such trouble. (Probably I felt the same way some developers feel about me when I struggle with CSS or cutting up a PDF or reading C–“how could you be a developer without understanding this?”.) I never talked to him about this, but it cemented in my mind why a new developer should learn the command line inside and out.
The command line interface (CLI) will always be available. Wherever you are (unless you are on an old mac, pre MacOS X, in which case, why?).
Unlike some of the skills I mention above, knowing how to navigate the command line will make your life easier as a developer no matter where you work or in what platform or capacity. It’s a general purpose skill, like understanding how to use version control or learning a text editor.