Build Plate Temperature Initial Layer refers to the temperature of the heated bed during the printing of the initial layer.
If Heated Build Plate is disabled, the material on the build plate will cool down faster. That's because the heat from the molten material can transfer faster to the build plate than to the insulating air.
Having the build plate heated slightly hotter while the first layer is being printed keeps the material away from the freezing point a bit longer, so that it doesn't get a temperature shock which causes the layer to shrink and warp.
After the initial layer is completed, the build plate temperature will be set to ordinary build plate temperature, but it will take a while for the build plate to reach that temperature.