Suppose some mod implements a speedbook for spells (if you aren’t familiar with the concept, it is basically a pocket book to read magic words from as opposed to a huge wizard’s tome from movies). Let’s say there’s only room for twelve spells in the book, one page per spell. The mod implements a new action to flip forward or backwards in the book, and a second action to begin incantations from the current page. It also adds actions to flip directly to each of the twelve pages/spells. That is a total of fifteen distinct actions.
Now a modpack author includes this mod in their roguelike pack. This pack puts a heavy emphasis on casting spells from the speedbook, and they disable a lot of other functions like mining. So the author wants to add default keybinds on the top row of your standard keyboard, that is the numbers 1-0 and ‘-’ and ‘=’ (‘[’ and ‘]’ on dvorak). These keybindings would flip to pages 1-12 in the speedbook. Right clicking might be bound to cast the spell, and scroll wheel up/down might be bound to flipping forward/backwards in the speedbook. On a controller, maybe the right and left bumper buttons will flip forward/backwards in the speedbook while one of the front buttons casts the spell.