[jdmichal]

You're not wrong, but it's also important to remember that they're running a heat engine to extract work. The amount of work that can be extracted is dependent on the temperature differential between the heat source and the heat sink. The water being cooled there is to keep the heat sink cold. And since the heat sink will always be colder than the heat source, they can't recycle any of that heat back to the heat source without expending energy to move it. Which would kind of ruin the point.

It's not that the heat isn't useful, or even that it's not a large amount of heat. It's that the heat is in the wrong place, and they need to efficiently get rid of it.