it is difficult to calculate internal energy U ,however ∂U can be easily determined why??explain with example

According to Einstein's equation U=mc² we can calculate absolute energy of a body,but in this expresssion the term c² reffered to the energy of any system which is quite large and measurement of this much amount of energy is impractical.Therefore it is more practical to measure differences in energy U, so we always measure differences relative to some arbitrary standard state.
For example-The internal energy U of a mineral can be determined as the sum of the potential energy stored in the interatomic bonds and the kinetic energy of the atomic vibrations.It is found that weakly bonded minerals have relatively low potential energy and thus low internal energy, and when a mineral is cold such that its atomic vibrations are slow it will have low kinetic energy and thus low internal energy. Internal energies are always defined relative to some non-zero standard state, so we typically talk about changes in internal energy dU which can be easily determined.