Calibers I own, I own for a reason. Often, it's a consideration of effectiveness/versatility Vs. availability.
.22 LR - rifle I got in high-school. Great for small-game hunting and pest control
12-ga - first gun purchased in my name. Apartment defense, versatility of multiple ammo options
7.62x54R - Rifles at big 5 for $70. Cheap ammo availability, good brush rifle.
7.62x39 - Cheap ammo for plinking, semi-auto, good all-round backup rifle. Bonus: already had cleaning kit for it.
.308 - more power than x39, detachable mags, better for hunting. Still same cleaning supplies.
.45 ACP - Needed a good handgun. .45 is a proven round, that is (in theory) widely available.
.38 Special - Sig. Other needed a self-defence gun, .45 was too powerful for her. Got the gun REAL cheap.