Power racks and squat racks are different from cable machines because racks support a loaded barbell, while cable machines create resistance through pulleys and cables. A rack gives the bar a place to start, finish, and be caught. A cable machine gives the user adjustable lines of pull from different angles.
The practical difference is barbell structure versus cable resistance. Racks are better for squats, bench presses, overhead presses, rack pulls, pull-ups, and heavy barbell work. Cable machines are better for rows, pulldowns, curls, pressdowns, rotations, and accessory exercises from controlled angles.