There is a higher learning curve, and we can argue about the tradeoffs you make, but some powerful tools can be difficult to learn to use. Complexity != bad design; sometimes you're just exposing an underlying problem space that can't be simplified without being cut off from part of the solution space.
Everything can be controlled through the keyboard, typing into every window does fuzzy search of its contents (and that window might contain a list of code symbols, a list of database tables, a list of search results, or many other things).
Every action can be bound to a key combo of your choice. Every interaction with the GUI can be stored as a macro, edited and replayed.
And so on, and so forth.
I'll also add that (like the parent comment) I did not get the appeal. Not until I forced myself to use it more and saw the benefits.
By the way, using a GUI doesn't automatically using the mouse for everything, think of GVim or Emacs. the problem with terminal emulators is the emulating part, where they are forced to comply with the idiotic rules from the '70s.