Why do I have a feeling that the purpose of this gifts is basically this (skip to 18:45) :
"Reciprocity refers to the non-market exchange of goods or labour ranging from direct barter (immediate exchange) to forms of gift exchange where a return is eventually expected (delayed exchange)"
So you know, you buy something to your friend (and spend real $ on it.) Your friend receives it a and thay are like, oh, it is so nice ! I should buy him / her something too !
...And the mechanism winds-up !