why are adjusting entries needed at the end of an accounting period?

Answer 1

Not all entries are picked up during the normal day-to-day activities. Some examples would be:

Allocation of prepaids and accrued expenses.
Addition of prepaids and accrued expenses.
Depreciation and amortization expense.

Also, sometimes an adjusting entry is needed to correct a posting error (wrong account, wrong split of principal and interest on a debt payment, etc.).