A belt is used to attach weapons and tools to the character. Items attached this way can easily be equipped with hotkeys.
The belt can be worn by the player since version 41. It offers two slots in the hotbar on the bottom of the screen.
A belt could previously be used in the since removed story mode, where it could be used as a tourniquet for your spouse, Kate Smith.
The health system is rumored to be improved further at some point in the future, likely giving some usage to the belt, specifically with the return of NPC survivors.
Belts are most commonly found in bedroom wardrobes and shelves in clothing stores. They may also be found in kitchen counters.
From items.txt (Project Zomboid directory/media/scripts/)
Weight = 0.3,
Type = Normal,
DisplayName = Belt,
Icon = Belt,