The only other case where TurboTax asks for the year-end value of your traditional IRAs is when you have made an excess contribution and TurboTax needs to determine the amount that is subject to penalty. The amount subject to penalty is the lesser of the amount of the excess or your year-end balance in traditional IRAs. However, your original question implied that Form 8606 is involved, so it seems that you have basis in nondeductible traditional IRA contributions and need to determine the taxable amount of a distribution as previously described, which requires knowing your year-end balance.
