The Ready Area is also a battleground between your creatures. This is where their attack and defense will come into play regarding your offensive and defensive moves.
A popular question amongst beginner's of the game I hear a lot is: "Do you have to attack if you have a Creature in the Ready Area?" If you are the first one with a creature out, then the phase occurs but nothing happens. However, if you and another player have a creature out, then an attack is mandatory. This is where more strategy can come into play when you decide from your rolls what exactly you want to summon to the Ready Area and how you want to go about the game.
Many creatures have abilities that can be used depending on other creatures in the Ready Area. For example, the Quaxos' Promos gain certain stats if an opponent has a Quaxos' Promo in their Ready Area. Their are several other possibilities, and I can guarentee you'll eventually find them all throughout a few played games.