I guess the following is grammatically correct.
Patricia is a chef. She is preparing the food for tomorrow's party.
The preposition "for" indicates the reason of her preparing, which is the party.
I guess I could also consider the party the purpose. In contrast, What should I called the food? Is it strange to call food the means to achieve a purpose? Are there any other nouns, which relate to "purpose" or "reason", could be used here?