Here's why:
* Prepositional phrases are groups of words that start with a preposition (like "in", "on", "at", "with", "to", etc.) and end with a noun or pronoun.
* "In addition" functions like a prepositional phrase, with "in" being the preposition and "addition" being the noun.
Example:
* In addition to her job, she volunteers at the local animal shelter.
Here, "in addition" is the prepositional phrase and acts as an adverb, modifying the verb "volunteers."