- For local development, this built the Rust library automatically
  when you invoked `yarn install`, whether you wanted it to or not.

- For the published package, this either did nothing (if the correct
  prebuilds were present) or produced a weird error (if they weren't).
  You can't use the published package without prebuilds, so maybe this
  was useful, but you'd find that out pretty quickly when loading the
  module failed.

- If you used the "link" feature to build Signal Desktop with a local
  checkout of libsignal, this would build the Rust library
  automatically when you `install`ed in the *Desktop* directory,
  whether you wanted it to or not.

- If you pointed Signal Desktop at a local checkout by copy instead of
  link, this would just fail, because the Rust parts wouldn't be
  copied with it.

Overall, it's simpler to just have this step be explicit.