Your cards act as a defensive wall covering your own HP, with the aim being to overwhelm your opponents' cards in order to get a crack at their health bar. Playing a card utilizes the requisite force gems - colored gems that your cards can actively or passively "harvest" by being in play. The rationale behind this is two-fold: it requires you to start with weaker cards, and build a set using cards of the same color.
But the attritional nature of the rules means that success is a numbers game - you'll find yourself hurling in any old card when you get the opportunity. And if a gung-ho approach is a legitimate tactical strategy, you feel that the rules haven't been tweaked as tightly as they might.