This is mostly useful when using the "Multiple SCMs" plugin, since a build may be triggered by changes in one or more distinct SCM branches. Currently, the change set list shows all commits in one list, and does not indicate where the commit originated. For the vast majority of build configurations, this would then start showing "Commit abcd123.. in origin/master by John Doe" – however if you use Multiple SCMs, or if you have configured the job to build all branches ("**"), the current list would not indicate where the commit came from.

I'm submitting a pull request that will add the "in ${cs.branch}" text to the change set log, if the branch is available.