Here's why:
* Interrogative: They are used to ask questions.
* Adverbs: They modify verbs, adjectives, or other adverbs, providing information about time, place, manner, or reason.
Let's look at some examples:
* Why are you going to the store? (asks the reason)
* When will you be back? (asks the time)
* Where did you find that? (asks the location)