The cash is used to buy upgrades for certain combos and more importantly extra lives, which will come in handy during the difficult stages. (thexboxhub.com)
I think it's because we were engaging a player's imagination — they would try to predict what certain combos would result in. (asteroidbase.com)
More powerful combos definitely require a bit of extra practice, but the combo challenge mode serves as a great way to learn certain combos for each character. (monstervine.com)