Cost is a property of individual enchantments, and is used by vanilla in combination with environmental aspects like tool enchantability and bookshelf count to determine the final cost of an enchantment as shown in an enchanting table.
Having access to the base cost of an enchantment using these vanilla methods can allow plugin developers to determine the "value" of an enchantment, and use it in custom logic where needed. I came across this recently when trying to assign an economic value to enchantments during tool repairing, and noticed these values don't seem to be obtainable under the current API.
