1. HR isn't there to be your advocate. The human resources department's function is to serve the needs of the business; its loyalty and responsibilities are to the company. Now, in some cases, that means advocate for employees against bad managers, because it's in the best interests of employers to retain great employees, identify and address bad management and stop legal problems before they explode. But plenty of other times, what's best for the employer will not be what's best for the employee, and the best interests of the employer will always win out. That's not cynicism; that's simply what HR's mission is.
2. HR isn't obligated to keep what you tell them confidential, even if you request their discretion. HR staffers aren't doctors or priests, and you shouldn't assume confidentiality when talking to them. If HR reps hear information that they judge needs to be shared or used to address a problem, their job obligates them to do that. In fact, in many cases they would be being professionally negligent – or in some cases, even breaking the law – if they didn't act.
Now, that doesn't mean that you can never talk to HR in confidence. But you should work out the terms beforehand explicitly – and should stay aware HR is still required to report certain things, like harassment or illegal behavior, even if they agreed to confidentiality before hearing your report.
3. HR knows things that they aren't telling you. Whether it's that changes to major benefits are approaching, or why some departments get significantly more resources than yours, or who in the organization is essentially untouchable, or who is on their way out, the human resources department learns things in the course of their work that they're not allowed to relay to you. If an HR rep who you normally know to be responsive is stonewalling you or seems resistant to explaining something, it's possible that they're simply not allowed to share something confidential. (That said, if you're regularly not getting what you need from HR, consider pushing back or talking to a different rep.)
4. HR's job is to support the company's managers, not to dictate how they operate. Some companies give HR more power than they should – such as letting them control how other departments hire or make promotion decisions. But in general, if you're a manager and your HR department is creating obstacles to your work (for instance, making it harder for you to hire great people or hire as quickly as you need to, or making it difficult for you to address performance problems forthrightly), you should push back. Escalate the situation, or find an ally higher up in the organization who can overrule HR or push for different procedures.
5. Your HR department might be great, or it might be awful. Some HR departments are tightly synced with the company's culture and goals and do excellent work – ensuring, for example, that managers are well-trained, benefits are strong and well-administered, salaries are benchmarked to industry and market norms and increased when needed, and that they help rather than hinder a company's managers. Others, though, focus more on holding office parties and then get in the way when managers need to hire, give feedback and handle sticky personnel issues. A good HR department can help a company get more done. A bad HR department will just get in the way.
But there's a huge amount of variation in human resources, so don't assume that what was true at a company you worked at previously will be true at your next.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.