Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. For example, "The book of poems" shows the relationship between "book" and "poems" - the book contains the poems.
* Prepositions often precede a noun or pronoun, forming a prepositional phrase. In the example above, "of poems" is the prepositional phrase.
Let me know if you have any other questions about prepositions!