First, you are seated, which disengages some of the hip flexor muscles so they don t assist the quadriceps as you press the weight. The back is supported by the machine and can therefore relax. The movement is guided and does not require the assistance of other leg muscles like the hip adductors or abductors to keep the contraction centered. Because of the assistance a machine can provide in securing the body into a position and guiding it through an exercise, it has some definite advantages. It is very useful for beginners and can teach the correct form and plane of movement involved in working a certain muscle.
Machines isolate muscles better than free weights, too.
This means they are very good at making a certain muscle group strong and developed. This is useful for those recovering from injury or surgery. You can usually lift more weight on a machine than with free weights and the weight is focused on one or two muscle groups rather than being dispersed among stabilizing musculature. Let's take the free weight equivalent of the leg press for example. The squat is a different exercise than the leg press because it incorporates more stabilizers into the movement and mimics real-life motion much more closely than an assisted leg press.