LinkedIn is a fantastic networking tool that can be used to stay abreast of industry news, discuss new trends with people in your field, meet people who can help further your career, and intelligently market yourself and your business to people who will want to meet you and use your services.
You should also be using it to help other people reach their career goals, because that's what social media's about - the more people you help, the more who will want to help you. If you're looking to change companies or jobs there's no better place to research how to make contact with people at those companies. LinkedIn will help you find the person in your network who can help introduce you. You can also reach out to people in your groups (if their settings permit it) by sending them a message and asking them to connect with you.
Do any or all of those things intelligently and LinkedIn can become the central hub of your career advancement. Do them wrong, and you can quickly become a pariah.