You might as well ask what good is Belief in a God or Gods ?
Neither belief or lack of it determines who a person is or how that person will react in society.
First you have to accept that mankind is just another animal species on the face of the planet and as such we are predominantly a herd or social animal who have a need to live in family or other social groups. We do this for protection and other social needs.
Most civilised society’s demand of their resident’s obedience to certain rules and a general acceptance of this leads most of us to behave in a social manner towards one another. Certain human traits like, greed, selfishness and jealousy make this social tolerance more difficult. Add in religious intolerance and you can add in mistrust and hatred to the mix.
How many true atheists do you suppose are involved with Jews and Palestinian’s killing one another, or Christians versus Islamists. Even in today’s news we have a bomb in Northern Ireland.... where we have Roman Catholic killing Protestants.... although this time I think the RC lunatics managed to kill one of their own just because he was a serving policeman !
I am a Pagan.... I have never once heard another Pagan claim my Sun is better than your Sun and gone on a killing spree !
Good people are good because that is how they are and how they want to be treated, it is how they were brought up by good parents to be good and constructive members of society. Religion only confuses that, it wants to take credit for it.... but it can’t... Religion preaches only ever taught one thing.... Acceptance or Death !