You are receiving the request for your beginning and ending mortgage principal balance due to the change in tax law related to the mortgage interest deduction. There are now different mortgage interest deduction limits based upon when the mortgage was originated and what it was used for, so portions of a mortgage listed may include interest that is not deductible. Therefore, the beginning and ending mortgage balances are needed to determine the deductible portion of the interest being entered. You can find your ending mortgage balance information on your statements or by checking with your mortgage company.
Here is more information about the tax law and the provisions for now being able to deduct mortgage interest: