Power-ups and weapons in Doom are implemented by giving items to the player. A weapon item is added to the inventory; a power-up item changes the player's state; ammo and health items modify player stats.
In addition to moving and "using", the third way the player can interact with the game is through items originally implemented as weapons. There is a single dedicated "fire" key to activate the current readied item, but the script system for what an activated item does is fairly generic and can do things other than fire weapons.
"JUMPWAD" modifies the activation script to cause the player to jump. It uses a series of complicated quirks and is rather limited in how it works. A dedicated "jump" action could free developers to implement a jumping item in new games without sacrificing other features.
An item could serve to accelerate the player like a vehicle while holding "fire". Releasing would constitute a braking system.
In a dialogue-driven game, using an item could constitute interaction with game characters.
A challenging platform puzzle could involve acquiring a levitation system. Firing the item would accelerate the player upward, and releasing would let gravity take over.
If one of the types of ammo represents money, a developer could implement a shop wherein the player points toward something to buy and presses fire to pay the requisite price and obtain another item.
An item could heal the player or allies on activation. The player could alternate between different spells. Ammo would constitute "mana" or "magic points" in this case.