For bank reconciliation, why are journal entries required on the book side?

Because you had changes to your company's accounts that have not been recorded like interest received or paid. The information comes from a bank statement, which means the bank has posted something in your account that you may or may not know about. For the items you did not know about you have to do an adjusting entry to record the item in your company's accounts so that your bank statement balance will be in agreement with your company's cash account.