XTB works as a mobile satellite app that allows you to sign-up and edit players for a tournament, as well as report results and scores for matches in progress. It is not designed to be a stand-alone system for running tournaments. Basically the head TO sets up the tournament in advance, and gives proper permissions to his/her team of sub-TOs (runners). Then the runners can use the app to register players for the tournament using the app as they walk around the venue. After the tournament has begun, runners can also use the app to quickly report match results and scores.
But you may be asking... why use the app, when you can already do all that with the websites for said third-party APIs? The difference is, XTB gives you additional access to register players and link them to their forum user IDs on forums which use the XenTorneo Ranking system (such as 8wayrun.com and Smashboards.com). By registering a player with their user ID, it guarantees that player will get credit for their performance at that event when it's submitted to the ranking system.
This alleviates extra work for the TOs after the tournament, and it establishes a sense of confidence in the ranking systems.