Well, try pushing someone away from you, or throwing a ball, without stepping into what you're doing. You have no accuracy.
Power means shifting body weight.
The weight is shifted, creating momentum, into your body's power zone to empower your arms or legs. You can shift your weight in a variety of ways, and instinctively tennis players are always trying to shift more weight into the ball. But what we want is a system that gets the most bang for the buck and can be repeated easily. All athletes are trying to achieve power with the least amount of energy expenditure possible because overdoing things leads to injuries and inconsistent results. I know biomechanists would add minutiae to my descriptions, but I want to keep it simple.