It buys attention for the issue. Many people (see other comments in this very post) refuse to believe these things.
And by resigning he no longer has to feel personally guilty for what happens.
Refusing to believe what things? Unsubstantiated allegations about fellow workers inner experience?
By resigning he's making room for someone with less moral scruples, or even just less awareness, to step in and continue the work without said scruples/awareness.