Hello Sandeep,
As I assume you have an understanding of what is Pro Rata and what is Based on Another Key Figure.
For why and when would you use these Pro Rata is used in cases where you have prior information on the proportion factors of the aggregate groups individual members. And you want to use this information to calculate the disaggregation for any changes at the aggregate level. This i would say is most common and simple method for disaggregation. One of business example would be to use it for already existing product lines.
However Disaggregation based on another keyfigure is used when you want to use the proportions calculated for an already existing set of data, and you do not have any prior information to have these calculated, or the proportions as per this prior information is not usable for some reason, in this case you would like to use the proportions calculated for some other data set and applied on this one for disaggregation. Business example could be new product line, needs to be introduced in a region and you want to use the proportions for already existing similar products for the customers in the region you would like to use this.
Regards
Abhishek