I'm a new people manager for a large company. My interaction with HR leads me to be believe that competent at HR , especially for large company, will give you a fair deal (but never a "good" deal) and does a shit ton of work I am thankful not to do myself, including a lot of what you mentioned. And, as a manager, I believe good intentions can align with professional responsibility. For example I sincerely don't want sexual harassment to happen among any of my direct reports. It also serves my company's interest, but it would be incorrect to assume my motivations were purely cynical or mercenary, because I truly believe in what I'm doing. I hope the same is true of the people who work in HR.

However, years of lawsuits and anecdotes reveal companies (and HR departments) are sociopaths even if the people working in them aren't necessarily: You're treated fairly because it is in their economic interest (retention) and legal interest (labor law, lawsuits) to treat you fairly, and unsurprisingly you hear the worst HR horror stories from people who work in places where they are easily replaceable or on the lower-wage end of overtime-exempt -- those external pressures to stay accountable don't apply.

It's good not to completely dismiss HR and I don't like it when anyone preaches absolute cynicism about their fellow humans, but it is basically good advice to remember HR helps you so far as you and they share interests (and if your HR department is corrupt or incompetent, all bets are off).