One of the purposes of identifying prepositional phrases is to help locate the subject and verb within a sentence, since neither is located within a prepositional phrase. One the phrase is eliminated, the subject and verb must be within the remainder of the sentence.
Another reason to identify prepositional phrases is that they often connect ideas and provide more information about verbs. The reader or listener has a clearer understanding of a sentence once the prepositional phrase has been spotted. In addition, the phrases frequently offer details about locations, people, objects, concepts, relationships and time. Knowing these supporting features broadens comprehension.