The working tree is where live edits exist before they become a commit. Checking it prevents accidental overwrites, unrelated staging, and confusion between saved history and local drafts.
ELI5
Think of Working Tree as a tool from the builder toolbox. The current files on disk in a Git repository, including edits not yet committed.
In practice
Use it when you are installing, editing, testing, reviewing, or shipping code. In practice, define the owner, input, output, and failure mode before you rely on it.