rustup makes it easy to have multiple Rust versions installed side by side, so you can test compatibility with older versions, or try beta or nightly builds (e.g. `cargo +1.77.0 test`). I think that functionality is rustup-specific and not easily replicable with tarballs or apt.
Apart from that rustup makes it easy to add and remove optional components, libraries for cross-compilation, and update Rust itself. These things could in theory be done with apt if Debian repackaged all of these components, but they didn't. Having one way to manage Rust installation that works the same on all platforms makes it easier to teach Rust and provide install instructions for software.
Apart from that rustup makes it easy to add and remove optional components, libraries for cross-compilation, and update Rust itself. These things could in theory be done with apt if Debian repackaged all of these components, but they didn't. Having one way to manage Rust installation that works the same on all platforms makes it easier to teach Rust and provide install instructions for software.