Why is accounts payable aging report needed for an audit?

I think you mean an aged Accounts RECEIVABLE report.

If you are familiar with auditing, the auditor needs to perform a risk assessment and satisfy certain assertions with regard to accounts. One assertion for Accounts Receivable (A/R) is Valuation & Allocation, which basically asks the question, "Is the account valued correctly?"

The auditor must perform tests and gather audit evidence to satisfy this assertion, one procedure includes examining a company's aged A/R report, which list the company's customers that owe money to the company. The report states which balances have been owing for different lengths of time, and normally an estimate is made to determine what values of A/R is uncollectible.

If the company has not made an appropriate adjustment to account for these uncollectible amounts, normally the auditor will suggest that an entry is made to do so, or else the account is not valued correctly. If management refuses the entry, the auditor would have to decide how their audit opinion would be affected.