Page "Enthalpy" Paragraph 39

The U term can be interpreted as the energy required to create the system, and the pV term as the energy that would be required to " make room " for the system if the pressure of the environment remained constant.

When a system, for example, n moles of a gas of volume V at pressure p and temperature T, is created or brought to its present state from absolute zero, energy must be supplied equal to its internal energy U plus pV, where pV is the work done in pushing against the ambient ( atmospheric ) pressure.