by Don  

The preposition за has multiple meanings and is used with several cases. One of those meanings is in sentences where someone goes somewhere to purchase something, where it is used with the instrumental case:

Я зашёл в киоск за сигаретами. I went to the kiosk for cigarettes.
Я пошёл в булочную за хлебом. I went to the bakery for bread.

The preposition doesn't necessarily imply purchasing.

Алёна зайдёт к маме за книгой, которую она забыла на днях. Alyona will drop by her mom's place for the book that she left there a few days ago.
Витя, забеги к соседу за гаечным ключом, который он у меня взял на прошлой неделe. Vitali, run over to the neighbor's place for the wrench that he borrowed from me last week.
Я заехал в клинику за вакциной от сибирской язвы. I dropped by the clinic for anthrax vaccine.

It can also be used for picking someone up on the way somewhere:

Я зайду за тобой в восемь часов, и мы вместе пойдём в кино. I'll come by for you at eight o'clock, and we will go to the movies together.
Она заехалa за мной в семь часов, и мы поехали к её тёте на день рождения. She came by to pick me up at seven o'clock, and we went to her aunt's place for [the aunt's] birthday party.

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