Here's why:
* Interrogative: It introduces questions.
* Adverb: It modifies verbs, adjectives, or other adverbs. It tells us how, when, where, or to what extent something is done.
For example:
* How did you get here? (modifies the verb "get")
* How fast is the car? (modifies the adjective "fast")
* How often do you go to the gym? (modifies the adverb "often")