Here's why:
* Adverbs modify verbs, adjectives, and other adverbs. They tell us *how*, *when*, *where*, *why*, or *to what extent* something is done.
* Prepositional phrases are groups of words that begin with a preposition and end with a noun or pronoun.
So, when a prepositional phrase modifies a verb, it's acting like an adverb, telling us more about the action of the verb.
Example:
* The dog ran (verb) through the park. (prepositional phrase acting as an adverb)
Here, the prepositional phrase "through the park" tells us *where* the dog ran, making it an adverbial phrase.