No two legs are created equal. Chances are, one’s stronger and/or bigger than the other, even if just slightly.
The goal, then, is to make them as close to identical as possible, to seek balanced development from side to side, which is where unilateral training comes into play. A leg-training program in which every exercise is done both legs at a time is an incomplete one. The legs need to work independently of each other on a regular basis, and doing leg presses one leg at a time is one of many great ways to do this.
The leg press is an exercise for all of the lower body muscles. Extending the knees against the resistance offered by the leg press works the thigh muscles - the hamstrings to the rear and quadriceps to the front. The action of extending the hips works the butt or gluteus maximus muscles. The load bearing effect of the leg press also strengthens leg bones: the femur, tibia and fibula. Leg presses also strengthens the knees and hip joints. The leg press can be used to develop strength and/or muscle size - called hypertrophy - depending on the type of training being performed.
Leg presses use a guided movement that minimizes your need to balance the weight and coordinate your limbs. This means you are free to concentrate on lifting and lowering the weight - a particular benefit if you want to train using especially heavy loads. Most models of leg press also have range of movement limiters that prevent the weight from falling and squashing you if you fail to complete a repetition. This means that leg presses are especially suitable if training alone. Leg press machines also provide lower back support which may reduce the risk of suffering a back injury.