What is the Use of Group Condition Indicator in Condition Type?

It is used for giving discounts on the basis of total material quantity in one material group.

For Ex: 

Product    Qty     Material Group 
A             200          01
B             300          01

Now, If discount on qty more than 100 is 1% and on qty > 450 is 3%, then individually they are eligible for 1% discount, but combined(same Material group) qty is 500 which is more than 450 and hence eligible for 3% discount.