Probably this is the virtual goods sale system which there is only in Japan.
Example: Doraemon Gacha in Ameba Pigg
And users can get a limited item as a reward, if six kinds of all items are got.
In Doraemon Gacha, users can get Doraemon parka. I yet get only three kinds :) If we can get Doraemon Gacha most cheaply, it is 1200 Ameba Gold(1200 yen). However, it is almost impossible. Users must play Gacha many times to get a limited item.
This is “Complete Gacha” (Kompu Gacha)". Ameba Pigg is a normal virtual world. So, the game system is not incorporated in a virtual goods. However, the social games are different. The system which can play a game advantageously is built into the virtual goods of a social game. Especially in a social game with a battle, this is an important element. Users play Gacha many times to get a limited item and use a lot of real money.
Why does the Japanese use much real money to a virtual goods? Probably it is because there is Gacha.