Here's why:
* Interrogative: They introduce questions.
* Adverbs: They modify verbs, adjectives, or other adverbs. They answer questions like "how?", "when?", "where?", "why?", and "to what extent?".
Let's look at examples:
* Why are you late? (Asking for a reason)
* How did you do that? (Asking for a method)
* Where did you go? (Asking for a location)