To perform the following tasks
Get a related value. For example, if your source table includes employee ID, but you want to include the employee name in your target table to make your summary data easier to read.
Perform a calculation. Many normalized tables include values used in a calculation, such as gross sales per invoice or sales tax, but not the calculated value (such as net sales).
Update slowly changing dimension tables. You can use a Lookup transformation to determine whether records already exist in the target.