Por is used to refer to a length of time (or duration); e.g. estaremos aquí por una hora ("we will be here for an hour").
Por is used to express gratitude; e.g. gracias por la ayuda ("thank you for the help").
Por is not used to indicate a destination; para would in used in such a case. For example, voy para la tienda ("I am going to the store").
Por is used to show movement through, along, or around; e.g. voy por la calle ("I go through the street").
Answer:
to indicate a destination
