A lot of good jobs never get advertised and are filled through word-of-mouth or internal appointments. Having a broad professional network to draw on can help you get a look-in on jobs that are going under the radar.
Contacts in your field can tell you about unadvertised jobs going in their companies. If you have taken time to nurture the relationship, they may even put a good word in for you and give you great insider information to help you with your application. Who knows, they may even end up interviewing you themselves!