Why Do We Need Optional Case Marking in Human Languages?

It's much more likely to have the case marking actually used when the agent is of lower animacy than the patient. And it also fits in with one of the ergative splits that we talked about way back when on our morphosynactic alignment episode. We expect things with high agency to be the ones doing things in the world, rather than the other way around. In those small number of languages that have optional case marking, you only mark the case typically when something unexpected happens.