Because it's for managing code, and the moment you get used to or want syntax highlighting and want to inspect or view diffs/patches, then you're in text-editor-like-GUI territory.
Beyond that, there's viewing and searching logs/history, viewing and managing branches and remotes and generally navigating the history of a repo. Graphical interfaces help with all of that.
A quick example that struck me was viewing the diff between two distant (ie, not adjacent) commits. With a decent GUI, it can be two clicks (with a modifier key). A really nice way of revising the history of a repo especially when there are multiple branches.