The sap tastes like water, not particularly sweet. It has to be concentrated 40:1 by removing water -- mostly by boiling. Charlie Williams demonstrates how they did that in Colonial days. But first, they would shorten the process by removing ice from the sap buckets (because the ice is pure water). The sweetness remains in the sap beneath the ice, so removing that ice saves some boiling time and some firewood -- and saves the farmer some time and some of the effort of chopping, splitting and moving firewood.