Buying ammo
Lots of reasons to keep a high inventory:
Firearms skills are easily degradable without constant practice. Practice shooting on the move, shooting at multiple targets, performing malfunction drills, shooting alternate/support hand, tactical maneuvers, could easily require 600 rounds a week to maintain proficiency.
Some people have guns in many disparate calibers.
In each caliber there could be subcategories:
a. Cheap steel cased FMJ for routine range work
b. but some guns are picky and require aluminum or brass cased ammo for routine range work.
c. Cheap JHP for running chambering drills; and high quality, premium bonded, expanding +P JHP for both training and self-defense carry.
d. Lighter pistol ammo (e.g., 200gr 45 ACP) for accurate, point of aim, long range shooting, vs. standard grain in heavier calibers for less than 100 yards.
e. 2 3/4" 12 ga shotshells for routine shooting; 3" magnum for training/defense, and some specialty stuff (dragons breath or smoke bombs)
f. Shooting .22lr in the neighborhood may require a silencer or shooting Aguila Colibri sans poudre so it doesn't bother the neighbors.
g. frequent bulk deals on corrosive military surplus stuff is great, but some guns you may not want to shoot corrosive in.
h. power ball for semi-autos picky with JHP
i. Super light and fast IQ for penetration/expansion diversity
j. Different core in calibers like smoking hot 7.62x25 for indoor vs. outdoor ranges
Also, in the last six years or so, there have been multiple times when ammo in certain calibers either completely disappeared, or became very scarce or expensive. Keeping a good stock helps to ride out the lean times. Its also a potential investment for the same reason.
So there is a niche for a LOT of different subcategories of all the calibers. With apologies to Chapter 3 of Ecclesiastes:
"To every caliber there is a season, and a cartridge to every purpose under the heaven"