Given that HR’s true use is to keep the company out of court, they protect against true worker abuse.
The HR department and a union can be a pretty effect team if both are truly working toward improving employment conditions. If both parties are working in good faith, an HR department looking after the interests of the company and the union looking out for the interests of the employees can negotiate some pretty reasonable compromises.
Unfortunately, many companies are too short-sighted to realize that improving employee conditions is often an outstanding investment. Sometimes, a union is necessary to force their hand to do what is really the right thing to do.
When in college I worked in a unionized sawmill on the weekends and during the summers. My father was the plant manager (#2 in the local hierarchy) after working blue-collar jobs for years at the mill. He was on the management negotiating team. I was pulled aside once by a union steward when some fairly heated collective bargaining negotiations were underway. He said that some of the union guys were talking about the management team and that they appreciated my dad's reasonableness because "he hasn't forgotten where he came from and he's willing to give us a fair shake." I was a pretty proud son at that point.