A sedentary lifestyle and aging diminish your resting metabolic rates. Usually, after ages 25-30, you can expect a continual decrease in muscle mass and resting metabolic rate and an increase in body fat. Strength training is one of the few ways to actually raise your resting metabolic rate. This is why strength training is important if you want to lose body fat. Think of it this way. Performing cardio is like driving a car with a small engine faster, while strength training like is putting a bigger engine in the car. So, however fast you drive, you will burn more fuel.
Strength training elevates your resting metabolism in two ways. First, the session itself causes a disturbance in homeostasis- basically, the muscle fibers try to recover. They restore glycogen, electrolyte balance, repair proteins, etc. This takes a lot of energy and time, up to 48 hours (longer in extreme cases). A significant amount of the energy to restore homeostasis is from fat.
The second way strength training elevates your metabolism by increasing muscle mass. The greater muscle mass requires more energy to maintain. Again, the more muscle mass you have the greater your resting metabolic rate. The increased rate has been reported to be in the range of 25-50 Cal per pound of muscle mass.