One of the main points running through the narrative of Absolved is that there are no obsolete weapons, there are merely obsolete tactics. Thus, a weapon which could kill a man a thousand years ago, such as a bow, a knife or a tomahawk, can just as easily kill a man today if properly employed. The continued utility of the bayonet -- a knife on a pole -- generations after it was first thought to be "obsolete" is but one example. Now here at the excellent blog StrategyPage is further evidence.
Preparations for war require large stockpiles of spares and munitions to sustain combat operations until factories can ramp up to meet wartime needs. When new weapons or equipment are introduced, many of these "war stocks" become obsolete, or sort of obsolete. In the early stages of a war, older equipment will often be hauled out of storage to help keep the fight going. So "obsolete in peacetime" is not the same as "obsolete in wartime". But even in peacetime, technological breakthroughs can suddenly make some gear cheaper to replace than continue using. More "wasted" inventory.
For example, during the 1991 Kuwait war, there was an unexpected need for deep penetrating bombs that could destroy underground Iraqi command bunkers. New bombs were designed, built, tested and delivered in less than two months by using barrels from obsolete 8 inch army artillery pieces to build the needed "bunker buster" bomb. According to the GAO, those old barrels were useless. To troops in wartime, no weapon is useless.
Many nations do this packrat thing, although the Russians are probably the worst offenders. They still have large quantities of World War II ammo and equipment in storage. Much of it was finally sold off when the Cold War ended in 1991. This was much appreciated by museums and private collectors. But they didn't sell everything, and even the U.S. has much vintage material sitting around, waiting for another opportunity. . . Demilitarizing (taking it apart and disposing of it) is expensive, so the tendency is to just leave the old ammo in the bunker and hope no one will notice.
But sometimes, the old stuff comes in handy.