Answer
Because the control account is built from day book totals (one source) while the personal ledger is built from individual postings (another), comparing the two independently prepared figures reveals errors. A difference signals a mistake in one of them—an omission, duplication, casting, or posting error—so the bookkeeper knows to investigate that ledger.
Real-world example A mismatch between the control account and the debtors list flags that an invoice was posted to the day book but not the customer.