No one's talking about perpetual motion here. Understanding the laws of thermodynamics allows us to build heat engines, which is how many of us got to work this morning. It also allows us to build nuclear reactors (or at least the steam-cycle part of it), which is how many of us get the power to write messages here.

What does our "understanding" of gravity allow us to do technologically? Nothing, because we have zero idea of how to manipulate it. The only thing we can do is understand how it works in the natural world so we can, for instance, navigate space probes accurately and get our GPS satellites to work. That's great and all, but it falls far short of the level of understanding that we have with thermodynamics and electromagnetism.