this is taken out of Bob taits BAK theroy book,
" the ISA is a set of atmospheric conditions which have been accepted as representing the average or standard conditions for the whole earth. Sea-level pressure in ISA is 1013 hectopacals (hPa), while sea-level temperture is 15 degrees C"
and you would use the ISA if you had to figure out the pressure hight. the equations pretty easy
(1013- QNH) X 30 + the height of the airfield. and thats how you figgure out the pressure hight (in feet) for your P charts