Aside from their designs, they have an utility in game. As you may know, when you win a battle you earn resources that can be used to craft cards, this happens through the rewarded battles. The rarer the spaceship, the most rewarding it is, depending on the number of daily rewarded games and the rewards multiplier.
This number will vary depending on the spaceship rarity as the following:
Note: when you consume all your daily rewarded games, you can still battle with your spacecraft, but you won’t earn any resources. This number resets everyday at midnight UTC.