Why are directed graphs important?

We have been reading about algorithms for MST, strong-connectivity, routing, etc. in directed graphs.

Also recently people have been doing research for dynamic and fault tolerant algorithms for directed graphs.

But I was wondering if there are any practical applications where the underlining graph network is "Directed". Other than social-networks all problems that I could think of like rail/road network, Internet network, etc. deal with undirected graphs only.

Edit 1: I understand that these can be used to model some scenarios where links are directed but I was wondering how often these scenarios occur in real-world, and how important is study of fault tolerance for directed graphs.

Recalling that a directed graph is a graph where the edges have an associated direction with them.

Using a directed graph you can represent asymmetrical relationships between nodes, while in undirected graph we can represent only symmetrical relationships.

Practically, using a directed graph you can represent:

• Road networks (using a directed graph you can represent streets' direction);

• hyperlinks connecting web pages;

• dependencies in software modules;

• prey-predator relationships;

• deterministic finite automaton.

Besides these classical examples, you can depict many other real-world scenarios (financial trade, scheduling, infectious disease, citation, control flow, etc) that needs an ordered relationship [1].

StackExchange Computer Science Q#68163, answer score: 38