The more merchants that take cards, for example, the more benefit there is to holding a card. And the more people that hold cards, the more benefit there is to merchants accepting cards. The network effect is both what can make it difficult for new systems to get off the ground and also entrench large networks.
The second feature of networks is that they involve people working together. In order to get the positive benefits from networks, there needs to be a commonality of interest among participants that are often competitors. When this is done well, it benefits both participants and end-users, and the wider community.