That how much will waste away or atrophy. This lowers your metabolic rate. If there is no corresponding reduction in caloric intake, the result is a gradual gain of body fat. Excess fat is unhealthy - and, to most people, unappealing. More muscle and less fat improves most people's physiques.
Strength training is the best way to simultaneously increase muscle and decrease fat.
There's a common misconception that to shed body fat you must do aerobics. Doing aerobics, it is thought, burns fat as fuel, while strength training utilizes carbohydrate energy. What's not often recognized is that it really doesn't matter what type of fuel your body is using. The bottom line is that it requires calories. That's why during strength-training exercise, even though the body relies largely on carbohydrates for energy production, you still lose fat.