Network routing information is useful for penetration testers in a number of ways. First, they can identify different devices between the penetration tester's machine and the target. The penetration tester can also glean information about how the network operates and how traffic is routed between the target and the tester's machine. Finally, the penetration tester would also be able to determine whether there was an intermediate barrier such as a firewall or proxy server between the tester and the target.
Kali Linux has a number of tools that provide network routing information.