I think anything that has physical {nodes} and {edges}, where all things in each category are alike but very distinct from the other category, and connectivity

I think anything that has physical {nodes} and {edges}, where all things in each category are alike but very distinct from the other category, and connectivity is only achievable via a limited set of (not free) paths. E.g. telecom networks, electrical grids, travel networks (including logistics), etc. All of these feature a very different node (usually a system or capital asset) vs an edge (usually a wire or constructed path), and insight into the structure is economically valuable. (That's talking to more traditional uses of graph theory, less-so to modern knowledge graphs)