How is this different from just requiring merge queue tests to pass?

`merge-queue-tests` isn't reported as a single status check by GitHub. Since it calls a reusable workflow (uses: ./.github/workflows/test.yaml), GitHub reports the individual jobs from test.yaml as separate checks, e.g.:

merge queue tests / build ucm (ubuntu-24.04)
merge queue tests / build ucm (macos-latest)
merge queue tests / transcripts (ubuntu-24.04)
merge queue tests / dll-ffi (ubuntu-24.04)
...etc
There's no single "merge queue tests" check to require. You'd have to enumerate all of those individually, and update the list whenever you add/remove an OS or job.

The gate job creates one concrete check (merge queue gate) that you can require, and it transitively covers everything via needs.