I'd argue all of the above. Humanity couldn't coperate and thrive without morality in general. Even if giving is its own reward, people want the reward; same variable, different object of it. The government definitely abuses the concept of justice which ultimately, only bears as much tangibility as there are people to have second thoughts before doing something questionable.

who says it exists?

g

I think many incorrectly correlate "Justice" with punishment. It is not.
Justice is essentially fair treatment of members in a society.
The world over, the terms of what constitutes fairness differs, is debated about, and changes over time.
However it's main concept remains the same.

Punishment exists for the selfish indulgence of an individual to feel vindicated -or- to feel morally superior.
Justice however, ensures that those who don't deserve punishment won't receive one unfairly; according to that culture/country's laws.

Is there a point to be made here?