While stationary trainers are used primarily to train for raw strength and power, rollers provide training for form and balance.
Because the bike is free to move and weave on the rollers, the rider must keep the bicycle centered while riding. You can't just space out like you can on a regular trainer. Imbalance between right and left legs will often cause you to drift one way or the other, so it provides some feedback to work on that. Pedaling at high cadence also tends to make people start to rock the bike, which will be amplified on the rollers, thus requiring the rider to focus and learn to keep their balance and form.
Regular rollers do provide some resistance training, approximately proportional to wheel speed, but not as much as a stationary trainer. They also don't appreciate sudden bursts of power in sprints, if you blast too hard you can come right off the rollers.
There are 'smart rollers' i.e. computer controlled resistance rollers, but just like dumb rollers, they are limited in how much resistance they can provide. Example is the Nero Elites, which I understand to be the best offering, can only provide 830 watts of resistance. Comparatively, I would expect even the cheapest smart stationary trainer to be well above that. 830 watts is plenty of resistance for just about everyone for steady-state training, but it's not going to work for all-out sprints.
When you are riding around on the street, try to ride right on a line, e.g. the white shoulder line, without moving off of it. See how far you can go. If you can't, riding rollers can be good training for that. It's a surprisingly difficult skills to ride in a perfectly straight line on a bicycle, especially under hard power. That's an important skill if you are riding in a peloton where constantly weaving back and forth, even a small amount, will both slow you down, and make the others around you nervous. With a few hundred hours practice on rollers, you should be able to keep both wheels right on that line virtually indefinitely.
Lastly, they are also a great source for comedic videos of people falling off of them. It happens to us all.