If you're flying from a private site, that is one without ATC, which QNH value would you use to fly cross-country?
One derived from the aircraft altimeter 'adjusted' from the airfield elevation setting, one published on the 'web' before flight or the Regional QNH obtained from ATC?
How would you determine the base of controlled airspace defined as an altitude? Maybe some don't care enough to bother? I remember having my Silver height attempt disallowed years ago because the instructor worked out, with the prevailing Regional QNH, I would technically have JUST been in controlled airspace - not so if I based the climb on the pressure setting for the airfield at the time (a bit tight that I always thought?!).
I might have expected this conversation on a private flying forum from an inexperienced pilot asking a genuine question due to lack of understanding, but on a forum inhabited by 'professional' pilots I find the subject surprising.
SITW
PS: Not to mention 'collision avoidance' issues!