This make rainbow be true rainbow not some girl you can freely buy in supermarket.
Every rainbow is precious not a trash or rubbish rainbow.
When you success pull rainbow that's mean a small earthquake will occur in your house LOL.

Nutaku know rainbow hard to get so they offer chance
for easy own your rainbow on their server to make player happy... maybe...

And I'm not sure... Do I already tell that the popularity on Nutaku site is payment popularity.