ABOUT Product Group Object and Its Use in Business?

Hi Gurus,

i have some doubts about Product Group. recently i have gone through mrp run at product group level. i am able to run mrp at product group level.

but i am not able to understand the concept clearly. i have below questions .

1. what is the object of Product group because we already have material group.?

2. there is no predefined parameters about product group, they will decide by users.?

3.what is the use of base unit of measure field in product group how it affects the planning of materials under the product group?

4. what is the use of aggregate factor, and proportion fields in product group?

Thanks&regards,

Mahesh.