Leg training for increasing metabolism. Fat is burned in muscle; it's that simple. Therefore, the more muscle we have, the higher our fat burning capability will be. The lower body is comprised of the largest musculature of the body. It therefore makes sense that performing strength training for the lower body will have an invaluable effect on metabolism (burning fat). I often get quizzical looks from people when I tell them that performing a million crunches will do nothing to burn abdominal fat. They are even more surprised when I tell them that performing strength training for the legs can have a massive effect on burning abdominal fat. If fat loss is a goal, then lower body strength training is an absolute must. Nothing else will have the same fat burning effect.
Developing muscular balance with leg training. Muscular balance is very important for many reasons. Proper balance enhances posture, lessens the chance of injury, and creates an overall pleasing look. Aesthetics may not be important to everybody, but if it is important to you, then leg training is a must. We've all seen the guys who only train their chest and arms and neglect their legs and back. Their backs are rounded over and they have tiny little sticks for legs. Personally, I have never found that look attractive. But even if aesthetics don't matter to you, neglecting leg training can also lead to structural problems. The less weight you have on bottom, the less support you have to hold the heavier weight that is on top. This can travel right up to the lower back and cause low back pain and stiffness. Believe me when I say that it is not worth it. Cover all your bases and train those legs!
Frequency of leg training. I highly recommend training the legs twice a week. This is important for several reasons. The legs can handle and need a varied amount of volume/ stimulation to grow, so it is difficult to fit enough training into one session. They also recover rather quickly, so spreading the workouts out in a Tuesday/ Friday fashion will give you plenty recovery time. Between the two sessions, I recommend a variety of different exercises and modes of training including free weight exercises (squat/ deadlift), machines (leg press/ hack squat), and functional training (TRX Lunges/ Medicine Ball Lunges). Incorporating variety is vitally important to ensure continued results.