You can expand them and compress them. And so pressure changes because volume changes, so pressure isn't constant. And if pressure changes, if volume changes, then PV work is done. In other words, the system does work to expand by doing work in the surroundings, to move the surroundings away so that the system could expand. Same thing with compression. Therefore, when you're dealing with reactions of gases, this equation holds the change in enthalpy is equal to a change in internal energy plus p times change in volume.