Why to have different guns in same calibre:
22lr
223rem
222rem
22-250
22hornet
And on, and on....
Why to have different guns in the same cartridge/chambering:
Example- 22lr
One for walk around small game hunting.
One for rimfire silhouette
One for hunter class bench rest
One for heavy class bench rest
One for f-class rimfire
One for your mate to use when you are plinking...... One for you to use while you're plinking.... One for you wife/girlfriend.... One for the kids to learn on..,.,
308win:
One mountain gun for trekking, light for stalking in steep country or long trips, bolt action.
One short, heavy(er) barrelled pump for knocking down lots of pigs quickly
One heavy varminter for picking off predators and scavengers at range over a baited carcass or at a water hole.
One for silhouette
One for full bore Target Rifle
One for F-class standard.
Mind you, all this would entail quite a bit of shooting! :p
Enjoy collecting your rifles.