The cluster is human brains all hooked up to each other. So the robots don't understand why human brains are so good at solving problems, but they still can exploit that utility in a way that makes us humans feel very not-special.
This was the original plot of The Matrix (the Machines were using human brains for parallel computing), but producers thought the movie-going was too dumb to understand it so they forced the Wachowskis to change it to generating power.
The human is happy that they're special, but then find out that they're simply used as a tool because of that. There's an extra sting because humans aren't being used for something cool and exciting, they're just brains in a vat calculating waste management routing.