When oozie launches a map/reduce job it retrieves the tracking URL of that job to display in its own UI. Previously this tracking URL did not change except when the job completed. Currently the URL starts out as N/A and then changes to the real URL once the AppMaster launches. This breaks oozie, as oozie expects to be able to get that information quickly and get back to processing other requests.
Because the web app proxy is now available we can maintain backwards compatibility by instead of returning N/A we can return the URL of the proxy for that application. This relies on the fact that the application master's UI will display the correct thing for the URI http://<host>:<port>/, which MR and others now do.