Sample use case:
We currently use custom pipelines to do "version bump" processing (e.g., git tagging, etc). This currently is a two-step process, where we update the version in a file in the repository, then invoke the custom pipeline through the web UI for the appropriate branch.
It would be much simpler for us if the custom pipeline could take arguments, for example through additional environment variables.
For example, we might have:
pipelines: custom: tag-version: - step: script: - ./tag_version.sh $VERSION
Then the existing "Run Pipeline for <branch>" window could have an optional "Add Environment Variables to pipeline run" which allows the user to specify additional variables beyond what's already configured in Settings/Pipelines/Repository Variables.
The use of environment variables is just an example here, and I'm not attached to that specific solution if there are alternative ways to pass information from the "run pipeline" UI to the pipeline script.