Rollers make very noticeable movements in your cycling that do not come apparent while riding on the road or using a trainer.

Rollers have little forgiveness so you develop a lot of balance while transitioning from positions and completing some movements.
A few examples:

Standing on the pedals. Doing it with rollers you learn to keep your line, minimize rocking, etc.
Reach for a water bottle, change hand positions in the bars, look back for traffic/riding buddies. All that while holding your line and balance.
One leg drills. High cadence drills. Smooths your pedaling while holding your line and balance.

Not an extensive list but gives you an idea. Besides all that, you get the exercising benefit as well. It takes a bit of time to get the hang of it but it does help improving your riding skills. If your goal is more fitness than technique I'd say the trainer provides results faster as there is no time spend learning. Good luck