How is this different to a shell script / command alias?
Sure you could accomplish something similar with direnv to maintain the project-level scope. But Just makes it more explicit.
I think looking at the features, you see "minor quality of life UX improvements", and it's not mind-blowing. (just commands implicitly run with the workdir of the justfile, just --choose uses fzf, soft tabs not required, etc.).
The effect does end up being a nicer tool to use.