More sales=more 3rd party support, more money for developers to create games and more money to spend on game development. not to mention more online players to compete with. Without sales, Nintendo, Microsoft, or Sony would not even exist.
Without at least decent sales, 3rd party developers would have no incentive to create games for that system, the company would have less money and thus would not be able to make as many games or have the money to create games that use the system's full potential.
Yet so many people say console sales don't matter. Why?