^exactly. Lash has a factory recommended threshold of .010" to .014", which is not much...but the cam lobes ride on parts that move a whole lot more than that in length, and move extremely frequently...just a slight amount of valve spring fatigue, and everything is out of whack...also, its important for any cam shaft to go through a frequency of stress that remains as uniform as possible along its length...lash out of spec on one valve, sends that resistance (or lack there of) down the shaft...and it will quickly build resonance that can destroy multiple things...
so its not something to mess around with...it has nothing to do with how 'aggressive' a cam shaft is, as this requirement spec is a function of the cam's base circle, not how big or small the lobes are...it is the measurement taken when no part of the 'toe' is in contact with the lifters, you could literally destroy a head with cams half as aggressive as stock...if the lash is incorrectly set...it has to do only with the fact that the shafts are integral to a system that has a whole bunch of moving parts...if you're changing cams, this procedure only adds about 30 minutes of work (way less to just check it, though)...and in most cases, it'll check out just fine...but the damage poor lash can cause is far higher than the handful of shims needed to correct it, and the time it takes to change them...