Dear Aternos,
In some mods or some plugins they use addons which adds more items/things to that mod or plugin your using on your world. For example, when you have the lucky block mod it can have addons like the pink lucky block or the rainbow lucky block which needs to be in a addon folder. I was thinking when you install forge or a software that uses plugins it allows you to access a addons tab which will have all the addons for mods or plugins. It will then put the addons like for the mods into a folder called addons and whatever folder addons for plugins are called. This will allow more things players and/or server hosters can have in their own server allowing more things to do with expansions of mods or plugins. I hope this does make sense!