Valve lash

Hi y'all:

I had a deja vu last week when I set my rocker lash. Back about 1969-1970, Mr Rouse's auto shop class, there was a slant on a test bed, I had to set the valves on it after they were placed out of adjustment. Holding the 7/16th(?) wrench on a moving rocker....I knew I'd done it before!
But here's a question, I set my Triumph Bonneville lash when cold, .003Int and .005 Ex, lash is taken up as pushrods expand. Why do we set lash when warm, and why so much lash? I remember reading a thread somewhere and the person said if the lash is too small, compression will get too high, this doesn't seem to make sense to me. It would seem to me that if there were only a couple of thou lash when hot, no valves would be held open and wouldn't get burnt, plus with a trace higher lift, breathing would be better. But what do I know!

Someone 'splain this to me!


TIA

Dan