While robots don't necessarily have the same kind of instincts or intuition that humans do, they do most faster and they don't tire, which means that in some situations robots might be more ideal than humans. Over in Japan, it seems that the country's volleyball team thinks so as well which is why they're practicing their spikes against robot blockers.
In this instance, the robots used are pretty straightforward. They moved in predetermined directions and have huge outstretched arms that will be used to block any spikes. The goal is to train the volleyball players to time their spikes so that they won't get blocked, but like we said the predetermined movements mean that it isn't quite like playing against a human.