I think the point here is that if $BIGCORP makes some unpopular compensation change that affects many employees (e.g. reducing the weighting of performance in t

I think the point here is that if $BIGCORP makes some unpopular compensation change that affects many employees (e.g. reducing the weighting of performance in the pay formula, reducing base pay relative to RSUs, etc.), a union would give employees a way to push back on those changes and/or negotiate for some other concessions in return. Actually, I imagine a $BIGCORP workers' union to be much more involved in this kind of negotiation than e.g. in demanding that wages be flat with no room for pay increases based on merit, which is the usual FUD that anti-union folks roll out in these conversations.