Racism is a lot older than a message board. Many of your ancestors were racist. But at some point they were won over, either by rational discourse or evolution.

Tribalism is our natural state, we gravitate toward those who act & look similar and away from the different. It takes thought and discussion to fight against this instinct. With kids growing up just being told racism is bad, many don't go though the intellectual work of actually proving it to themselves.

Seeing these arguments in public is important. It may help convince someone living in a racist bubble why its wrong. But I doubt seeing some racist joke or tirade will actually convert a nonracist person to racism. On the other hand a deleted post does absolutely nothing.