Learn Spanish (Latin America): Te doy la receta para que prepares la cena. - I am giving you the recipe so that you prepare dinner.

Because it follows para que (so that). In Spanish, para que is always followed by the subjunctive mood because it expresses an intention or purpose that hasn't happened yet, rather than a stated fact. Here, prepares is the tú form of the present subjunctive.

Could I just say para preparar instead of para que prepares?

No, because there are two different subjects in this sentence. The first person (I) is giving the recipe, and the second person (you) is preparing dinner. When the subject changes, you must use para que plus the subjunctive. You would only use para plus the infinitive if the same person were doing both actions, like in Tomo la receta para preparar la cena (I take the recipe to prepare dinner).

What role is Te playing at the beginning of the sentence?

Te is an indirect object pronoun meaning "to you." It tells us who is receiving the recipe. In Spanish, these pronouns usually go right before the conjugated verb, which is why it comes before doy (I give).

Can I translate para que prepares as "for you to prepare" or "so you can prepare"?