Many integrations between CIs and git frontends (as in showing green checkmarks under your pull request) only work properly if you push. Rebuilding manually in the CI will run the build, but it will no longer be recognized as related to a PR

The only integration I've used like that is github workflows and only because the project owner won't turn on the feature to allow manual re-run . Fortunately transient failures are rare on that project, but I still find it rather odd.

Because there are plenty of people working with systems you don’t know then.

I've had caching problems when triggering manually resolve on a new commit.