@glitzersachen @doctormo The UK.
For example, an employee, not necessarily knowing who's best placed to answer a query, might go to their union rep rather than their line manager or HR to ask "How do I ...? How does ... work? What would happen if ...?" and if the union happens to know the right answer then that saves HR and/or management time. (Of course the union will signpost them on when appropriate, eg if they don't know the answer or it's not their business to give it.)
@glitzersachen @doctormo Particularly if the union rep is on the shop floor, the line manager is always in meetings, and HR is on a different site.