Should prior year IRA distributions contribute to the year-end balance?

When entering the year end (2018) value of all traditional IRAs, TT states that I must include any IRA distributions made the previous year (2017) and goes on to say, "The amount you enter must be at least the amount of your distributions." What role do previous year distributions have on the year-end balance of the current year?  For the past several years, my total TradIRA contributions have been reported as distributions on 1099-R when they are converted to my Roth IRA. Each year-end TradIRA balance is zero. Why must I include the amount of the distributions in the year-end balance?