Ionization Energy and Electron Affinity
This is a practical matter of making sure that there is a common basis for comparing values from all different types atoms and molecules. If it were not the case there would be a range of different values for the same type of atoms or molecules. 

Depending on temperature the ionisation energy would be less due to a molecule's rotational and vibrational energy, and vary with temperature because the number of levels populated depends on the temperature. For some types of atoms there may be low lying electronic levels that could be populated at higher temperatures, say at 1000 K in a high temperature gas. Again the measured ionisation energy would depend on temperature.

Also, if this was not done the $\Delta H$ values would in effect have double counted the effect of temperature, once in the ionisation energy and again in the heat capacity term.