Rainbow Dash was very generous to give Rarity some of her mane.

I think that it's one of the keys for that box. The way they are going to get keys is by spreading lessons of friendship to people who need them, after which they get an item that has to do with it. At the end they'll get a crisis that happens to need the pretty box. Presumably it's going to be kinda like the dragon scroll from Kung Fu Panda, in that it doesn't give anything except for an idea and that the villain doesn't understand.
Then skittle-magic.