Souvenir. Not that it's necessary, but some folks collect 'stuff'.
In my early teens, I lived down the street from a Vietnam Vet. Very nice guy, but trouble like a lot of vets at the time. He had a nice collection of memorabilia. He had a belt of 0.50 inch (12.7 mm) rounds for a machine gun, some hand grenades (practice and combat), a bazooka (IIRC 5-6 inches, or 12.7 to 15.2 cm), a light anti-tank weapon (LAW), an AK-47, a practice 200 lb bomb, and a host of other munitions and weapons.
One of his buddies accidentally discharged a gun (IIRC, it was the AK-47) in the house. The bullet went through the ceiling of the first floor, through his room on the second floor, and exited the roof of the house. The FBI and ATF paid a visit and confiscated most items.
I spent a lot of time discussing his experience in the Vietnam War. It was ugly. Nobody should have to witness what he did.
I also learned a lot about military strategy, guerilla warfare (including IEDs), explosives and demolition. At the time, I had expected to war to continue and I was several years from draft age. I was able to read various Army field manuals (FMs).