It would be useful for integration with other tools, such as BrowserMob, dynaTrace, New Relic, and just about anything else that needs to correlate traffic from a browser to some particular "grouping". The addCustomRequestHeader forces an additional header to be bundled up with any browser request that goes through the HTTP proxy, which can be activated with the "addCustomRequestHeaders=true" browser option (as well as the captureNetworkTraffic=true option). A concrete example of this is that it allows dynaTrace users to use a custom JUnitRunner, which will automatically add a custom header for each test case that runs. The custom header will be consumed by dynaTrace, allowing dynaTrace users to see the performance profile of each Selenium test.