In their documentation regarding pull request pipelines, bitbucket says:
Pull requests: a special pipeline that only runs on pull requests initiated from within your repository. It merges the destination branch into your working branch before it runs. If the merge fails, the pipeline stops.
So I'm wondering, why merging before running the pipeline? Why not just running against the coming branch without merging? Could the reason be detecting merge conflicts early on in the pipeline before the real merge?