How does "back" function here? I don't know if it's necessary in this sentence.

Rebecca

"Back" in this case refers to 'behind' or 'the back of something'. In this video it could refer to the back of the building or an area that is not the main section. In the clip, "back" is not really necessary to the sentence, but it implies that it is an unusual location to be selling hats. Ex: "Let me check if we have any extra snacks in the store room.....Sorry, there is nothing back here!" Ex: "You shouldn't be back here, this area is restricted to staff only!"