Is there any logical reason why the ICE needs to run when travelling on the highway at highway speeds? This may have been covered before, but I couldn't produce anything while searching.

On the highway portion of my normal commute there are several large downhill sections, some approaching a mile in length or maybe even more, where the vehicle can literally just coast down without losing speed. Why does the ICE need to run here? I'm just envisioning the amount of extra fuel being burned for no logical reason at all.

Or am I just overthinking the whole thing....?