I understood it to be a political accounting gimmick to make the total cost of the tax cuts seem lower, since the standard way to measure this cost is to use an (arbitrary) 10-year time window. Under this approach, tax deductions that would previously be claimed in year 7 would now be spread out through years 7-11 and thus wouldn’t be counted. This is also why most people seemed fine with undoing the change, because it didn’t matter. But maybe I have this wrong.