What is `git checkout --orphan` used for?

It's used by e.g. GitHub Pages, which stores a repo's website inside the repo but on a separate branch. There's no reason to store anything but the website's history on this branch.

The core use for git checkout --orphan is to create a branch in a git init-like state on a non-new repository.

Without this ability, all of your git branches would have a common ancestor, your initial commit. This is a common case, but in no way the only one. For example, git allows you to track multiple independent projects as different branches in a single repository.

That's why your files are being reported as “changes to be committed”: in a git init state, the first commit isn't created yet, so all files are new to git.