Because it is faster, has great visualization and most importantly magit has great fallbacks. You can still use the command line from magit! Pressing "!!" in a magit-status buffer will get you there, while still showing you all the nice info in the other buffers. I use it for "git submodule update --init --recursive" for example.

I have seen people struggling with git log and understanding which commit is on which branch and all that. Of course you can use something like "git log --decorate --graph --color --oneline --all" or even alias that. In magit I do: "mag-stat" (Emacs completes this to "magit-status") which one does usually anyway, to see where the repository is at and whether there are uncommited changes and so on. When I need the graph, I do: "mag-log" "-" "c" "b". Then I can go to any line in the graph and check it out, because it becomes the default for when I type "mag-check" (completed to magit-checkout) and press RET. On command line I would be copying commit ids and branch names and would remember to use my alias for the long git log command.

Edit: Another great feature is how easy it is to commit only some hunks, instead of everything that has changed. Commiting hunks on command line is tedious afaik. You would have to specify row numbers.