Can you explain why auditors must obtain an understanding of the clients accounting system?

The auditor should review the client accounting system to gain an understanding of the flow of transaction and the specific control procedures to be able to make a preliminary evaluation and identification of these aspects of internal control on which it might be efficient and effective to rely in conducting his audit. It may be useful to trace a transaction through the accounting system to assist in understanding that system and its related internal controls.

because this will help the auditor to access the risk associated to the client due to software such as in appropriate calculations.