Why does a company prepare a bank reconciliation statement ?

To know real Cash Balance (Difference between Bank Balance & Book Balance)

We can ascertain following;

Cheques issued but not collected

Cheques deposited but not cleared

Bank charges debited by Bank & not accounted in our Books of Accounts

Errors, if any.

1. To be certain that the amount of cash reported on the company's balance sheet (and the balance in its general ledger Cash account) is the correct amount. The additions and deductions on the bank statement are compared (or reconciled) with the items that are entered in the company's general cash ledger.  Some differences, such as outstanding cheques and uncredited cheques, are noted as simply timing differences.
2. Since most companies use double-entry accounting or bookkeeping, any omission or error in the company's general ledger Cash account also means that another general ledger account will have a corresponding omission or error. For example, if a company had wired money from its bank account for emergency computer maintenance services and had not recorded the credit to its Cash account, it is also omitting the debit to the account Computer Maintenance Expense. The bank reconciliation could prevent this company from issuing an incorrect balance sheet (incorrect Cash and incorrect Retained Earnings) and an incorrect income statement (expenses would be too low, net income would be too high).
3. Performing a bank reconciliation results in improved internal control over the company's cash if done by someone other than the employee(s) handling and/or recording receipts and payments. Having another person reconciling the bank statement is known as the separation or segregation of duties and it should reduce the odds of dishonest acts involving the company's cash.