Prepositions are small words that are related to  another element in the sentence. They are essential because they provide  additional details about the sentence by locating events, people, and  objects in a place and time.

To locate an event in a time frame and add information about its duration, we use the prepositions for and since.

https://open.books4languages.com/english-a2-grammar/chapter/for-and-since-with-present-perfect/