When a horse gets cast, they get stuck and can't get up again, which (if they panic) can injure them by all their thrashing about trying to get up again. They can go into shock or exhaustion if their thrashing around getting nowhere. Less panicky horses can normally be rolled over without many problems, providing your careful. I don't really know why they get cast, I guess its them rolling in their stable when its too small or their too close to the walls.
As I understand, anti-cast rollers give them more leverage to roll themselves over again and become uncast. They'd need one if theye were 'recurrent' casters. It can be dangerous for not only the horse but the owner as well, because if your horse is thrashing around he could catch you in his panic and do some serious damage.
One cure is to turn them out, and not stable them as some horses can't cope with enclosed spaces as well as others. Also, banking the bedding around the sides of the stable walls helps prevent them becoming cast.