Strength training equipment works by creating resistance that the user must control through a movement pattern. Barbells, dumbbells, racks, benches, plates, cables, and machines differ in how they load the body, guide or free the path, and let the user progress over time.
The practical explanation is mechanical. A rack changes setup and safety options, a barbell links both hands to one load, dumbbells allow independent paths, plates control load jumps, benches change body angle, and cables keep tension through a pulley path.