How do your employees communicate and collaborate? How do you interact with vendors, remote sales staff, freelancers and peers? If you don't have a single collective space where your employees can go to find anything they need, then you're not maximizing your business' use of the Internet.
Intranets done right are more than a corporate facebook or a collection of HR documents. A proper intranet allows makes life easier for employees, collects and puts institutional knowledge at everyone's fingertips, endears employees to your organization and simply works with what you already do every day.