well the very basic reason, is to keep the valves from floating, during higher than stock rpm (over 3k rpm), and or when the exhaust brake is being used to keep the valves seated. the stock springs can start to float at around 3200 rpm, with the 60lb. springs, i have seen 4k RPM, many times. they are a necessity so you dont cause damage to your valve train, and or even psiton damage.
wes
edit: if your planning on doing a 3 or 4k govenor spring kit, you need to do both intake, and exhaust valve springs. but only the exhaust springs if stock govenor springs, and a exhaust brake is the setup.