Network effects
The value of a network increases as the number of users does (Metcalfe's Law)
The value to any user (or node) depends on the number of other users they can communicate with. Therefore the total value of the network is driven by the total number of users squared. This means network effects follow an exponential curve—slow growth for a long time, followed by an "elbow" and steep ascension.
Another valuable effect of networks is solving the last mile problem (where the cost of the "last mile" of delivering a service is usually responsible for a majority of the cost). Building one network to deliver services to multiple nodes is vastly cheaper than delivering those services individually.
See: Systems