While on a regular bicycle a plastic bag over your saddle may do, motorcycles require a better solution to qualify as a good saddle cover.
A saddle cover protects the saddle of both scooters and motorcycles against rain, snow, dust and sun. They form an indispensable accessory for those who use their motorbike daily and in all kinds of weather, for a commute or other reason. You simply place the cover over your seat and take it off just as easily.
A good saddle cover is also a solution when your saddle is not completely watertight, due to worn seams or cracks and tears in the material. There are few things more unpleasant than sitting on a saddle of which the foam is soaked to the limit, wetting your trousers as soon as you sit down.
For certain vehicles there is a specific saddle cover. But as saddles generally come in a limited variety of shapes, the universal saddle cover usually does a very good job.