Since the Nash equilibrium is not always the best joint decision, why is it important?

There is something curious to me in the Prisoner's dilemma. The (mixed) Nash equilibrium is $\{(0,1);(0,1)\}$. In that notation, I mean it is the mixed strategic profile $(s_1,s_2)$ where $s_1$ and $s_2$ assign the action Silent to $0$. But if the prisoners could make a joint decision, the profile $\{(1,0);(1,0)\}$ would be the most suitable. Then I'm wondering in which sense the Nash equilibrium is more interesting that the last profile. In general lines, it is easy to find important applications of Nash equilibrium. But I would like to compare its importance to the one of the profile that represents the best joint decision profile, that is, the profile that provides recompenses with the minimum variance possible and the maximum sum of gains. If someone could help, I'd be grateful. Thanks in advance!

• A Nash equilibrium is important if we want to model our players as selfish: who cares about recompenses with the minimum variance possible and the maximum sum of gains when you want the biggest payoff for yourself? If you can improve your outcome unilaterally, why wouldn't you? – Omnomnomnom Jun 16 '17 at 13:46