A subsidiary ledger contains the details to support a general ledger control account. For instance, the subsidiary ledger for accounts receivable contains all of the information on each of the credit sales to customers, each customer's remittance, return of merchandise, discounts, and so on. With these details in the subsidiary ledger, the Accounts Receivable account in the general ledger can be a control account. As a control account, it will simply report the aggregate amounts of the accounts receivable activity.
By having the details of the accounts receivable activity in a subsidiary ledger, a company can better control its financial information. For example, the credit manager and others in the credit department of a company will have access to any and all of the credit sales information through the subsidiary ledger without having access to any other account in the company's general ledger.
In job order costing systems, the job cost sheets or records serve as the subsidiary ledger containing the detail for the general ledger account Work in Process. The Work in Process account is now a control account containing aggregate amounts for direct materials, direct labor, factory overhead applied, transfers to finished goods, etc. Manufacturing personnel will have full access to the job cost sheets without gaining access to other accounts in the general ledger.
Since companies are integrating accounting records with their other information into one database, I assume there will be less use of the term subsidiary ledgers in the future. There will likely be a report generated to provide the information formerly contained in the subsidiary ledger.