The sane answer is that it solves a cooperation problem.

Reciprocal altruism sometimes sends a relatively weak signal - it says that you will cooperate so long as the "shadow of the future" is not too ominous.

Invoking "good" and "evil" signals more that you believe in moral absolutes: the forces of good and evil.

On the one hand, that is a stronger signalling technique - it attempts to signal that you won't defect - no matter what!

On the other hand, it makes you look a bit as though you are crazy, don't understand rationality or game theory - and this can make your behaviour harder to model.

As with most signalling, it should be costly to be credible. Alas, practically anyone can rattle on about good and evil. I am not convinced it is very effective overall.