Circuit training provides a dose of both muscle training and aerobic conditioning in one workout. They have built-in safety features that reduce risk. When you can no longer push a weight, you can simply slide out from under it. Machines remove balance as a consideration. With free weights, the novice must learn to balance the weight while exerting force.
Machines isolate the muscles that are being exercised.
This is an advantage when exercising to rehabilitate an injury or strengthen a specific weakened body part. Machines are self-contained and neat. There are no weights scattered about, creating a hazard and an eyesore. In favor of free weights Free weights are the choice of muscle builders. Some machines are closer to free weights in their effects, while others are much less effective.