"Belts don’t provide tremendous protection, but they are often imbued with mystic abilities. They can also store much-needed potions and keep weapons hanging in easy reach."— Game Guide(src)
In Diablo I, belts are not equippable items, but rather a panel where one may put up to 8 scrolls and potions for quick use (1-8 digits on a keyboard). There is also a special button assigned for quick use of health / mana potion, which first uses items from inventory and then from the belt.
Belts are present in Diablo II.
Belts play an important role in a character's survival. Designed to provide quick access to items, most belts contain multiple slots, loops, and cords from which to suspend potions and other vital items. The larger a belt then the more potions and scrolls it can store. Wearing a sash or larger belt expands the initial four-slot capacity of a character's belt by adding one or more extra rows of four slots. While only four slots are normally displayed on the Interface Bar, a player can access the additional slots by highlighting any of the visible belt slots or by pressing the tilde ("~") key to toggle the belt open and closed.
- A player can drink a potion from your belt by right-clicking on the potion. Also, each occupied belt slot has a number, from 1-4. Pressing one of these hot keys is a fast way to drink a potion without having to click on it.
- Larger belts have other advantages. When a player drinks a potion or reads a scroll, the item above it drops down to replace the consumed item. If one has a potion or scroll in a hot key slot and you pick up another of the same type, the new item automatically stacks above the first one. This continues until the column is full. If there is no more room on the belt, the item is placed in a player's backpack.
- Health, Mana, and Rejuvenation potions are automatically placed in the belt when picked up. Players can also place scrolls of Identify and Town Portal, as well as the other drinkable potions, in their belt. However, items like scrolls are not automatically placed in a belt unless there is a matching item in one of a player's four hot key slots and there is an empty slot above the one with the matching item.
- Holding down SHIFT and RIGHT-Clicking, when buying Health Potions, auto-fills any empty slots in the belt. This also applies to buying scrolls of Town Portal and Identify.
- Holding down SHIFT and LEFT-Clicking on Health, Mana and Rejuvenation potions in inventory, will auto-move them to the appropriate column in one's belt.
- Holding down SHIFT and a Hot Key (1-4) or Right-Clicking on Health, Mana and Rejuvenation potions in the Belt, will give the Potion to a player's Hireling if the player has one.
Belts return in Diablo III. While they no longer serve to store potions (in game, but still do lore-wise), they offer a high range of protective stats. Offensive stats are only found on Legendary belts.