Like you'd expect? No one is making claims of free energy. It's like putting a stirling engine on the side of an internal combustion engine. You have a chance to grab some of the waste heat, why not use it? Specifically, capturing that waste heat will boost efficiency by some small %, if the additional systems total construction cost is less than the efficiency given over the lifetime of the system, why not?

It's just converting between two forms of energy: total energy is still conserved. You might be thinking about the second law of thermodynamics, which limits how much can be converted.