You learn to use more of your existing muscle mass when exerting force. A dumbbell exercise can be altered simply by holding the dumbbells with the palms facing forward, or facing the thighs, or facing the rear. This results in three separate variations, each working the muscles in a different way. Machines are limited by their construction with each machine designed to allow only one exercise. Some machines have multiple workout stations, but these are actually many individual machines combined into one large and complex unit.
Free weights are relatively inexpensive.
Such machines also might not provide smooth and true movement patterns and might require considerable upkeep. Resistance training requires a high level of desire and motivation. Muscular strength is an important attribute in many sports, and resistance training is an integral part of the preparation. Free weights allow you to lift anywhere you choose. On a nice day, take them outside or do your exercises on the back porch.