The p90x app makes it easier for you to do your workouts when you have time. Since the workouts usually take over an hour it is handy to able to do them anytime you need. There are a couple benefits of having this app on your phone. The biggest is you have access to the workouts on your iPhone or iPad. The second benefit is that you can track your workout progress right on your phone. This eliminates the need to keep track of your workouts on the workout sheets.
There are a couple disadvantages with using this app. The first is that it is extremely expensive for an app, coming in at $4.99. The second is that even if you have purchased p90x, you will still need to purchase the workouts on the phone. You can get the bundle but it will set you back $59.99.
The app is one of those convenience apps that do make life a little easier. The big selling point for me was being able to track the workouts.