People do business primarily with people they know and like. Resumes and cover letters alone are often too impersonal to convince employers to hire you.
Job listings tend to draw piles of applicants, which puts you in intense competition with many others. Networking makes you a recommended member of a much smaller pool.
The job you want may not be advertised at all. Networking leads to information and job leads, often before a formal job description is created or a job announced.