As has been said in answer to the OP ..it's convenient to have loaded multiple magazines.,
especially at a range where time is money and would rather spend it shootin instead of loading
I used to shoot 2500 rounds in a few hours of target shooting with my .22 semi auto rifle and luger pistol.
-shrug-
It was the effort to do the best I could ..self competition.
Paid off in spades when I joined the USAF
..first time with the M-16 and every time I had to qualify thereafter I shot expert.
Didn't 'like' it as much when they switched to the .22 adapter setup in the M16 ..felt too much like a toy.
Wife and I decided no guns in the house when we had our first child ..never got back into it.
Too many other things kept me busy altho every year I got to go play on the range while I was in the USAF.
Had a friend who used to think anything less than 35,000 rounds was 'getting low' and he had to restock
Also had a friend (Harry Wells) who owned and operated a WW2 vintage .50 cal machine gun.. among others.
Different strokes
Funny story:
Harry decides to shoot his .30 cal machine gun like John Wayne at a 'shoot' one fine day.
Loads a belt up, takes aim ..and lets fly.
Well.
The gun only cycles 3-4 rounds and stops ..and Harry screamed like a schoolgirl.
First ..the recoil was damped too much by Harry not being able to hold the gun steady enough..the reason it stopped firing.
Second.. the barrel traveled upward even in that short a burst to the point where the last round was almost straight up as Harry fell on his butt.
..and last but not even least, and funniest ..and why Harry screamed ..
That way cool belt feed system with the brass clippy dealies ..as it fed the rounds in..
..it yanked hairs right out of Harry's arm.
That whole bare-arm superhero John Wayne stuff just does not work in real life ..yanno?
Harry told me it worked fine when he backed up to a tree and put on a long sleeve denim shirt
-Frank aka GE