The reason is that it serves as a criteria to check that you entered all positions correctly.

So when creating a bank statement, you usually have to enter manually the opening balance and closing balance. After that, you enter all the positions and when you press compute, the system will calculate the result of all entered transactions. you can then compare it with the closing balance before validating.

If you validate the bank statement, the system will throw an error when there is a difference between the computed result and the manually entered closing balance