What are the sticker packs used for?

Usually, the game makes up events that reward you with a particular item that might serve as "make-shift" currency. For instance, "The Battlefield Service Center" event rewards players with "Activeness Points", which you can use to purchase items there too. There is another event too that rewards you with "Posters". However, different events have different mechanics so you should always take the time to understand how things work or how each event differs from one another. Hope this helps!