Most obviously, during the period when the balance sheet is running down, if the economy encounters adverse shocks, it may be appropriate to commence the reinvestment of principal payments again in order to preserve conventional policy space if the federal funds rate were to drop below some threshold level, perhaps similar to the “well under way” threshold.
More broadly, although the two tools can achieve roughly similar effects, they are different, and we cannot rule out that there may be special circumstances in which these differences may be particularly valuable.