When you meet someone face to face, not only can you get your message and information about your business across, but over time you will build up a trust and rapport. Once you get to a trust point, then the people you network with are more confident to refer business to you.
This is where people start to recommend you. Its a lot harder to get to this trust point through advertising and social media for example, although it is possible, but takes a lot longer and can cost a lot more than networking.