Here's why:
* It modifies verbs, adjectives, or other adverbs: It tells us *how* something is done or to what extent something is true.
* It expresses contrast: "Nonetheless" indicates that something is true despite something else.
Example:
* "The weather was terrible, nonetheless, we had a great time."
* "Nonetheless" modifies the verb "had" and shows the contrast between the bad weather and the good time.