The reason why ionisation energy is measured as the energy required to remove 1 mole of electrons from 1 mole of substance is that it allows the ionisation energy of different substances to be compared. This is because the ionisation energy of a substance is going to be different in different states.Therefore you can't compare the ionisation energy of a substance in a solid state to the ionisation energy of another substance in the liquid state. For this reason, the ionisation energy of all substances are measured in the gaseous state. It is like the standard solubility of a substance is taken at 25 degrees. It doesn't make sense to compare of the solubility of one substance at 25 degrees and the solubility of another substance at 100 degrees.
Now you might ask why did they choose to measure the ionisation energy of substances in the gaseous phase, why not the solid or liquid phase? The reason why ionisation energy is measure in the gaseous phase is because in the gaseous phase, there is very little attraction between other particles which effects the ionisation energy. Therefore, if the ionisation energy was measured for a substance in the solid, since the particles are close together, the intermolecular forces will distort the value of the ionisation energy.