Motor scooters usually offer a bit more protection against wind and rain, because of their fairing and construction. On the other hand, the rider of a scooter doesn't want to put on a full motorcycle suit every time, just because these (often urban) motorcycles are most frequently used for shorter trips.
The rain apron is the ideal solution when the weather lets you down. Your legs get a lot of extra warmth and they will certainly stay dry. You attach the apron around your waist, as an alternative to rain trousers. Some models are lined to make sure they are useful in winter.
Only condition? Holding the bike between your legs is impractical, if not impossible. So for motor scooters only !