Lancaster has the right perspective!
Firstly a "magnum" is a bigger/more powerful version of an existing calibre. True H&H may have coined the phrase with the belted .375 H&H; certainly much more powerful than the .375 Velopex which correct me if I'm wrong gents was the original belted case? - it was used for headspacing not extra material at the case head to hold back higher pressures.
Then came the arms race with the 9.3x62! This set the cat amongst the British pigeons & boy did they deliver. The .375H&H, .416 Rigby, .404 Jeffery,.425 WR just to name a few!
So our German friend Wilhelm Brenneke decided to up the ante - aka 9.3x64mm......Sorry gents but it tops the .375H&H.
Not to be out done Mr Weatherby made the .378 Weatherby Magnum & the velocity then went through the roof!

Now this is where Grandveneur is correct...."A good question were what do we need really and what is superfluous , sometimes ridiculous !"
We all like certain rounds, sometimes just for a name (Rigby), a calibre (.425WR) or a legend (Bell's 7x57mm) - it really doesn't matter what calibre we like, magnum or otherwise; superfluous numbers of rifles(sorry if I could afford more I would - & yes I certain agree with Rignymauser - a .577/500 Magnum would be neat!
So there gents - you are all correct - its all perspective!

But the 9.3x64mm is the best design & more efficient than the .375H&H....sorry folks I'm biased!
93x64mm