It doesn't nearly go all the way there though. Why do people need to use a command line and a gui tool (usually) for git? Because it's fundamentally not written to be used with a GUI. That I think is one of its biggest flaws. Using a GUI with git always feels like you are missing vital information and just trying to poke a cli underneath to do what you want.

Some design decisions also shine through like "no branch is more important than any other branch" which is completely mental considering how people actually use git.

Most of the guis are crap because everybody who builds them thinks that a GUI should just be a more or less a visual representation of the command line.

"Why do people need to use a command line and a gui tool (usually) for git?"

You don't. The reason is that you're using a tool that didn't budget the time to directly work on git data files and it uses the command line under the hood, because that's a hard business case to make for most small tools. This is not fundamental to git; the very top-end git-based tools like Github or Bitbucket all do their own internal, direct implementation of git functionality for this reason. It's not a characteristic of git, it's a characteristic of the GUI tools you're using.

A perfectly sensible one based on perfectly sensible engineering tradeoffs, let me add; no criticism of such tools intended. Git's internals from what I've seen are not particularly difficult to manipulate directly as such things go, but you are simply by the nature of such a thing taking on a lot more responsibility than if you use the command line-based UI.