It's funny, everyone thinks HR is about dealing with people, whereas it's actually about processing them. Processing entitlements, processing grievances, processing legal documentation, processing disciplinary procedures. Your line manager's the one with a personal relationship with you.

Mind you, quite a few people think an HR officer is the right person to go to with a problem. This is a _very_ bad idea. HR's job is to protect the firm, not the employees (sometimes this involves pushing out someone senior, but that's unicorn-level rare). If you want someone to represent your interests at work, it's time to unionize. :)