The Calculate Area function either calculates the area of just the selection or all active layers or clipping area. The result is stated in the unit you choose under Start » Change Units and can either be mm2 or mils2. Furthermore, the percentage is stated in relation to the area of the whole board which is important for one of the following use cases:
Calculating the total amount of soldering paste required
Activate your solder paste layer in your layer stack up and deactivate all other layers
Select Start » Calculate Area, choose "all selected layers" and press "Start"
You get the area of your soldering paste layer and only need to multiply it with the height of your template
Prevent Twist and Bow
Activate all signal layers in your layer stack up and deactivate all other layers
Select Start » Calculate Area, choose "all selected layers" and press "Start"
You get the areas of copper on each layer. In this case the percentage of these areas is important as it is advisable to get an approximately symmetrical layer stack up of percentages to prevent a so-called twist and bow of your curcuit board caused by an unbalanced copper distribution on your layers.
