Networking is an effective job strategy for two major reasons.
First, it allows you to get access to jobs that may not be made public. It is often the case that firms will not really advertise jobs right away. They might do informal searches through networks. They might ask their top executives if they know of people who would be good for a particular job. If you are part of such a network, you might find out about jobs that aren't even advertised.
Second, networking makes you more likely to get noticed. If you actually know some of the people who are going through resumes you are more likely to stand out when they see your resume. If you know people in various firms, they might put in a good word for you with the people who are choosing whom to interview.
In these ways, networking can be an effective strategy for finding a job.