Here's why:
* It shows the relationship between a noun or pronoun and another word in the sentence. For example, in the sentence "I went to the store with my friend," "with" shows the relationship between "I" and "my friend."
* It usually comes before a noun or pronoun.
* It can also be part of a prepositional phrase, which includes the preposition, its object, and any modifiers.
Let me know if you'd like more examples!