If you want to call ruff directly, this doesn't change anything. It's a purely optional feature.
However, to answer the question generally: people want this for the same reason that most people call `cargo fmt` instead of running rustfmt[1] directly: it's a better developer experience, particularly if you don't already think of code formatting as an XY-type problem ("I want to format my code, and now I have to discover a formatter" versus "I want to format my code, and my tool already has that").
If you want to call ruff directly, this doesn't change anything. It's a purely optional feature.
However, to answer the question generally: people want this for the same reason that most people call `cargo fmt` instead of running rustfmt[1] directly: it's a better developer experience, particularly if you don't already think of code formatting as an XY-type problem ("I want to format my code, and now I have to discover a formatter" versus "I want to format my code, and my tool already has that").
[1]: https://github.com/rust-lang/rustfmt
Some of us prefer well packaged tool that does everything instead of stitching together bazillions of dependencies.
Or maybe some prefer random versions of dependencies being downloaded and running over our code?