The idea of optimization of a network occurs in many areas -- so in a way your question could be addressed by looking at the differences and similarities between these fields:

Networks in electrical power transmission (complicated by capacity and special well-known laws that govern electrical flow). Typically there are non linear effects and needs to model various generation sources and sinks

Networks in pipeline systems -- must consider hydro rules and constraints,

Networks in transportation -- must obey conservation of flow and perhaps deal with capacity and cost,

Networks in social systems -- must consider behavior of agents,

And so on .... my own interest in networks considers hubs in airline (passenger and freight) systems

In the Quantitative Methods for the Management Sciences Course Notes of the Carnigie Mellon:

http://mat.gsia.cmu.edu/classes/QUANT/NOTES/chap11/chap11.html

the authors indicate that "Network optimization is a special type of linear programming model".

and they conclude that:

1) Networks are an important subclass of linear programs that are intuitive, easy to solve, and have nice integrality properties.

2) Problems that might not look like networks might be networks.

3) Networks provide a useful way to think about problems even if there are additional constraints or variables that preclude use of networks for modeling the whole problem.

This document can help clarifying the concept and applicability in many fields:

http://mat.gsia.cmu.edu/classes/QUANT/NOTES/chap11.pdf