Setting your Virtual service into routing mode turns ServiceV into an API or JDBC service recorder, building up the actions and endpoints in your Virtual service automatically based on real-world traffic. This allows you to avoid creating Virtual service manually, even without a descriptor! The faster you build your Virtual service, the faster you can build great tests and ship that awesome application.
Toggling the routing feature allows you to quickly test and diagnose issues in real time. Especially useful in agile and DevOps shops, being able to model, code, and integrate becomes a whole lot easier when you have ServiceV in the mix. Also very useful on an shared service (Virtual service deployed to VirtServer for use by others) when you need to visualize and diagnose problematic service calls from a partner, 3rd party developer, or even simply another member of your team.