1. Discoverability. It'll display the contextually relevant options and commands at most points. By using magit, you're learning the git CLI commands at the same time, including commands that you'd normally never come across without a comprehensive read of the manual or release notes.
2. Fewer keypresses. Also, extra shortcuts for some common operations.
3. The bits that CLIs aren't very good for: staging and unstaging pieces of files, viewing conflicts.
(I do still use the Git CLI a lot of the time too.)