I know it is pretty hard to measure the change in volume in open systems. Accordingly, calculating the change in internal energy using the first law of thermodynamics is pretty hard also. Is this the only reason for introducing enthalpy ($H$) as a new thermodynamic potential to make use of the constant atmospheric pressure, and to calculate the change in enthalpy ($\Delta{H}$) as it is equal to the in/out quantity of heat ($Q$) in this case?
If no, does the absolute value of enthalpy have any physical significance?