Adjusting journal entries are accounting entries that are made at the end of a period to record the various transactions and events that occurred during the period and are not yet recorded in the company’s books and accounts. Adjusting journal entries are needed to ensure that books and accounts comply with the accrual method of accounting and ensure appropriate recognition for revenue and expenses. The purpose of making such entries is to match expenses and revenues with the period to which they relate and, at the same time, record transactions that have occurred prior to the end of an accounting period but have not yet been booked in the books.

Adjusting journal entries are part of the process of closing the books at the end of a period. They are done at the end of a period to make sure that all transactions that took place during the period are recorded. These entries can include accruals, deferred expenses and revenue, depreciation, and amortization.

Accruals are expenses and revenues that occur in one accounting period but are not paid or collected until a future period. Examples include interest income, wages earned but not yet paid, and services rendered but not yet invoiced. Deferrals are expenses incurred in one period but paid in the next period. Examples include prepaid expenses such as insurance premiums and rent. Estimates are allocations or assumptions made about future income or expenses. Examples include depreciation expense, bad debt expense, and uncollectible accounts expense.

Adjusting journal entries are essential for an accurate financial picture of a business and its performance. For example, if a company incurs an expense in one period, but the expense is not recorded until the following period, then the company's financial statement would not correctly reflect its performance. The adjusting journal entry would ensure the expense is recorded in the period it occurred, giving an accurate representation of the company's performance.

In conclusion, adjusting journal entries are used to ensure that all transactions that occurred in a period are recorded and that companies adhere to the accrual method of accounting. They are necessary for an accurate financial report of a company’s performance and ensure that expenses and revenues are recorded in the period in which they are incurred.