It allows one to do aggregate calculations such as sums, averages etc. It is unlike expression transformation in which one can do calculations in groups.
Values can be calculated in single row before writing on the target in this form of transformation. It can be used to perform non aggregate calculations. Conditional statements can also be tested before output results go to target tables.
This happens when it faces DD_Reject in update strategy transformation. Moreover it disrupts the database constraint filed in the rows was condensed.