Why assign weight? to the banner and how rotator works using the weight?

If you assign a weight to the banner and choose the type of rotator using weight, banners within the rotator will appear at the defined frequency scales.

For example, if you set the weight of 2 X banner, and the banner Y weight 8 is 2 times out of 10 X banner appears, and the other (eight) cases will be displayed on the banner Y.