The Update Strategy transformation is an active transformation. When you design a data warehouse, you need to decide what type of information to store in targets. As part of the target table design, you need to determine whether to maintain all the historic data or just the most recent changes.
For example, you might have a target table, T_CUSTOMERS, that contains customer data. When a customer address changes, you may want to save the original address in the table instead of updating that portion of the customer row. In this case, you would create a new row containing the updated address, and preserve the original row with the old customer address. This shows how you might store historical information in a target table. However, if you want the T_CUSTOMERS table to be a snapshot of current customer data, you would update the existing customer row and lose the original address.