The reason is that English is a Germanic tongue with Latin rules of grammar. A couple of centuries ago, scholars who appreciate the beauty of English wanted to formalize its rules. As funny as that may sound, formal English with its Latin rules is a beautiful, flexible, and precise language. Using formal English will l give you more freedom of expression than will using relaxed English. Now, let's return to our discussion of prepositonal phrases.
Prepositional phrases are either adjectives or adverbs.
When they are adjectives, they modify nouns and pronouns just like regular adjectives do. When prepositional phrases are adverbs, they modify verbs, adverbs, and adjectives , just like regular adverbs do. This is important when forming subject-verb agreement because the object of the prepostion must not be confused with the subject or the subject and verb will not agree. Another thing to remember about prepositional phrases, is that they can have two or even three objects. Prepostional phrases are also important because they form one of the two ways possession is shown in English.