Here's why:
* Determiners specify which noun is being referred to. They come before nouns and are often followed by adjectives.
* "Both" indicates that there are two of something, and it modifies the noun that follows it.
For example:
* Both cars are red. (Both modifies the noun "cars")
* Both of them went to the park. (Both modifies the pronoun "them")
Let me know if you'd like more examples!