The problem also occurs if your CI Build + Test steps take a while to run, even on a small team pushing dozens of commits per day.

Two code-conflict-free changes may pass a pre-merge build+test cycle independently but may logically break one another if both changes are merged into master. Using a submit/merge queue guarantees that each change has passed tests with the exact ordering of commits it would be merged onto. The example described here is a better explanation: https://github.com/bors-ng/bors-ng#but-dont-githubs-protecte...