In what situation might you have to calculate the surface area of volume of a solid?

One application of volume is determining the density of an object. Assume the object is made of a pure element (eg: gold). If we know the volume (v) of the object, and we know the mass (m), then we can use the formula D = m/v to figure out the density D. Knowing the volume is also handy to determine if the object can fit into a larger space or not. Another application is figuring out how much water is needed to fill up the inner space of the 3D solid (assuming it's hollow on the inside).

The surface area is handy to figure out how much material is needed to cover the outer surface. This material can be paint, paper, metal sheets, or whatever you can think of really. A good example is wrapping a present and the assumption is that there is no overlap. 

Just turn the whole gym into pie