What would the scientific principle be when the purpose of the lab is to learn and practice techniques and calculations for determining volume and density of a substance?
It could a number of things only known to your teacher and/or the author of the printed instructions for the lab.
It sounds like an experiment to determine the density of a material. When you measure the mass and the volume of an object, you have the data needed to calculate its density. The relationship you use is:
Density = Mass / Volume