There are two departments I do my best to become familiar with, even friendly with, as soon as I start at a new company. The first is the IT department (which sometimes wraps under my responsibilities but I am referring to the situation where it doesn't, here). Without a cooperative and friendly IT staff, you don't have the equipment needed to run a development or engineering department. The second is the HR department (which almost never is under my responsibility!). HR is vital to my running an efficient department for obvious reasons: I have a staff of educated professional engineers, and almost always need more, replacements, or help with personnel issues. That's when a friendly neighbourhood HR professional is a god-send.
I depend on HR primarily to help me find new talent. They take the job descriptions or requirements I create, post them to where ever the company posts such things, talks to recruiters, and generally gets the word out that we're looking for some skilled people. When the resumes start rolling in, I depend on HR to willow out the useless ones and pass the rest on to me for further triage. Some companies with larger HR departments will do preliminary interviews or work with some of my team to narrow the search down, but in smaller companies I usually get a stack of resumes to troll through to find good fits. This isn't a trivial task, by the way. A typical posting for a developer can generate dozens of resumes. Each takes a few minutes to go through for a preliminary yea or nay, and then the most promising need a second pass to narrow down a few choices for further interviews or consideration. When the HR person knows specifically what I need and can understand the terminology, they often can ensure that I save hours and hours in winnowing resumes.
A good HR person can help enormously in the recruiting process even if they don't understand the technical requirements. After all, I doubt many HR people can understand when someone spouts on about Linux kernel components, or the intricacies of garbage collection in a language, but they almost all have years of experience in spotting baloney. And out of a hundred resumes for a position, easily a quarter are people who don't really have the skills I am looking for but are hoping to bamboozle and learn on the job! Going through those resumes is a total waste of my time, and that's a great advantage to having good HR people.
More than recruiting, HR is a good feedback loop. Often, if someone is unhappy in my team, for example, HR knows about it before I do. I've often had quiet conversations with my HR staff about "someone doesn't like..." without specifically identifying the person. A good HR person can separate the legitimate complaints from the unfounded ones and decide to help accordingly. Those quiet conversations, when they do occur, are useful to me because I may not have been aware of an underlying problem and can proceed to rectify it. HR is also a good bouncing point when I have issues with someone. I've had quiet talks with HR professionals about an individual, explaining the issue I've had with them, and see if they have some advice. This is particularly useful when I am relatively new to the company and don't have the history with the staff that the HR person may have.
A good, competent, logical HR person is a god-send. I've been lucky in that in every company I've worked with in the last decade has had some great HR people (Denise, you stood out!). Friendly, energetic, sensible HR people save my sanity and my time, and they tend to also be very nice, to boot. So, I make the effort to always be in synch with HR as much as possible. They make my job so much easier.