It's not, you can literally do everything this tool does with Git, and 80% of the features could be replaced with commands in your shell rc file also using vanilla git.

This tool was described perfectly the other day. JJ is the Dvorak of Git. Most people could careless about Dvorak layout, 99.8% of people use qwerty just fine. Those 0.02% though, they're loud, and they want everyone to know how great the reinvention of bread is.