NETGENERATE allows to create networks in a very comfortable way. For some small-sized tests of rerouting strategies, tls-signals etc., this is probably the best - read "fastest" - solution to get a network one can run some simulations at. The clear naming of the streets also eases defining own routes. But networks generated using NETGENERATE are of course useless as soon as you want to simulate a real-world network.
Using NETCONVERT one can import road networks from several sources, among them VISUM, shape files, and OSM databases. This is of course the tool to use for setting up real-world networks.
Still, most examples within the data-section were written as XML files by hand and converted using NETCONVERT for several reasons. At first, the examples are small enough and one may steer the output better than when using NETGENERATE. Furthermore, defining own networks using XML-data is more flexible.