Sometimes. The meanings are different. To "harness" means "to use in one specific way." The literal idea refers to work animals. A harness is a set of straps you put on a horse or ox. A horse is a powerful animal. Without a harness, it does whatever it likes. To "harness" a horse is to put a harness on it. You can attach it to a wagon. Now you can use the power of the horse to work for you, pulling the wagon. I use a car to travel in. I don't "harness" a car. Gasoline, outside of a car engine, is a dangerous substance. It burns or can explode. A car engine uses gasoline. But in this case we can use a phrase with the word "harness." The car engine harnesses the power of gasoline to do something useful for me, just we can harness a horse to do something useful for us.

In a few specific situations, yes. It's mostly used when talking about sources of energy, such as the image of harnessing wind and wave power. Remember that it is an image - it's like putting a horse into a harness to channel and control its running power.

You sometimes can, but why would you want to?

What do you mean?