As already said, the apps provide the smarts that control the trainer. But the other factor is time. Without the apps you enter a time warp where each minute gets stretched out to an eternity from falling into the black hole. The apps can control that by displaying something on a screen that is meaningful to look at, and that brings time back closer to reality. It may still be stretched by a gravitational wave, but at least it advances.

As for rollers, I don't know why that gets brought up every time someone asks about a trainer. Trainers are easier to set up, are easier to ride when distracted, and have better control. Rollers may be great if you want more "biking" feel, but riding the trainer is an end to itself.