The skills are there so that Alexa can interact with some 3rd party apps. So for example, if you use Our Groceries for your shopping list and you want to be able to tell Alexa to add stuff to that app's shopping list, you enable the skill for it and then you can just say "Alexa ask Our Groceries to add milk to the shopping list".
It is slightly cumbersome but it does enable 3rd party stuff to play nice with Alexa fairly easily.
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