This machine gets poo-poo'd by the strength training community. The thinking is that everyone should be able to do regular dips with their bodyweight, bare minimum, for a set of 20.
I don't disagree with that. If your shoulders are in good health and you've got adequate anterior serratus strength, then dips are a staple in a "just barbell" program. Dips do an amazing job of developing the chest and triceps and, for the most part, have a low degree of injury potential. (I write that even though I suffered a tremendous pec tear doing dips, but I still consider them relatively safe.)
The benefit of the Gravitron or any assisted dip machine is that the shoulder joint isn't dealing with as much torque on it by having to offset the load of your bodyweight. With the Gravitron, you can still get the benefits of dips, but in a more joint-friendly fashion that also allows you to focus on making the muscles do the work, rather than focusing on moving your body through space.
Thanks to the offset loading, you get a chance to "feel around" with different body positions, and that can help you figure out a torso and elbow angle that feels natural for you.
Lastly, you can crank out some bad-ass metabolic stress work with assisted dips. Start by selecting an offset load that allows for 12 reps. Then increase the offset load so that you can do another set of 12 reps, but this time to failure. Increase the load one more time for another set of 12 reps to failure.
If you haven't done a set of these, then you're missing out on some sweet pecs/triceps pump action. These hurt in a really glorious way.
If you're truly maniacal, hop on over to the Smith machine and do a set of Smith machine push-ups with the bar set at the bottom position. If that doesn't smoke all of your pressing muscles to the max, then you're truly dead inside.