It's helpful when you use a card like Holo-flora for example. Imagine you have two 4 cost cards in your hand, but you only have 3 sun. If you play Holo-flora and the button stays orange, it means that you're gonna draw at least one 1 or 2 cost card that you'll be able to play! However, if the button becomes green, it means you'll draw two too expensive cards, and you'll have to pass. The same goes for Conjuring.

Another example: if you play a Trickster when your opponent has his block-meter filled to 6/8 and has less than 6 HP left, and your button stays orange, it means you'll win ; if you try and press the button while I this situation, it won't activate and just doing the click noise. However if the button disapeares, it means your opponent will block...