Predictability and adaptability. A backhoe is good for very few things. In the same way, the Curiosity is only good at what it was designed to do. It can't adapt.
Humans, on the other hand can take the tools provide and experiment outside of pre-planned parameters. If something unexpected comes up, we have to build a whole new machine to deal with that and then we have to send it there. IN ADDITION, we can't just send a machine that does just one thing because that's terribly expensive, so we have to wait until a variety of EXTRA test labs can be added to the machine to bring down the cost-per-experiment to reasonable levels.