Inexperienced "Eastern" hunters on here who hunt elk with magnum cartridges have included shrapnel, rosco, Dober,JohnnyB,John Burns,greenhorn,Alan Day from the past....and a host of others I can't recall at the moment....but that'll do to make the point.
Most of these guys are from Montana so I guess that's the East...except Alan Day who was also from Oregon as I recall.
Most all of these have used standard cartridges as well and understand the differences.
I figure if you have to ask "why use a magnum?", you might not have enough experience with anything to answer your own question.
Just another horseshidt thread.