A significant problem for people with low end systems is achieving a good frames per second rate. The classic way to do that is to manually change settings. The issue with that is that X-Plane deliberately hides most settings. The workaround is to use an application that exposes those deeper settings. But 3jFPS adds another option. You set a minimum and maximum frame rate and the applications adjusts settings dynamically to achieve it.
I like the idea, but it seems to me that it has one major failing. I don't understand the idea of having a minimum and maximum frame rate, and I think 3jFPS becomes lost in that. I would far rather set a minimum and 3jFPS manipulate settings to make sure FPS stays at or above that. That doesn't mean that 3jFPS has no value, especially for people with low end systems, but at this time I don't see the point of it for a medium to high end system, because often settings are lowered to maintain a frame rate significantly higher than the minimum.