Record keeping for the most part. You need something to attach the CI jobs to and PRs are a good choice. There are plenty of cases were a deep review should be preformed. Always presenting that opportunity is good instead of having to do something different from the submitters point of view.
CI jobs trigger on the change, not the PR creation, so there's no need for the extra step. There's no need for a PR for trivial stuff. In the context of teams "internal to a company who know and trust each other", when deep review is still important, is the PR process useful, or is it busywork? Why not have the deep review happen during the development, rather than after?