Say two PRs get sent in by two different developers. CI builds both, passes them.

Reviewer comes and approves PR#1, it is merged into master. Depending how much CI power you have, PR#2 (and any other PR in queue) can be automatically built again on top of new master to verify it wont break anything.

You can do it lazily too and just re-check PR#2 once it gets approved by human - like other comment mentioned, CI coordinates and serializes all merges into master making sure master is always green.