The difference between bands and free weights is the type of resistance each has. When lifting weights, gravity plays a big part. You get more resistance during one part of the movement (lifting against gravity), but then gravity makes lowering the weight easier.
The fact that elastic resistance does not rely on gravity is that it provides continuous tension to the muscles being trained. When using bands with weights the resistance is constantly changing, so you have to work harder.
The type of resistance with bands is called linear variable resistance which just means as you stretch the bands further it gets more difficult. What this means is that, as the range of motion of the exercise increases, the resistance provided by the elastic equipment increases. As the bands length increases (from being stretched), it provides more resistance. One of the benefits of this is that as the range of motion increases and the resistance increases, the number of muscle fibers being used in the exercising muscle increases. This causes you to use more fast twitch muscle fibers, the ones that grow fastest, strongest and biggest.
In fact, according to a study by Dr. Jim Stopanni, workouts that incorporated bands and free weights together showed 2x more strength on bench press, 3x more strength on squat and 2x more MUSCLE MASS!!!
There are other benefits that are not offered by free-weight alone resistance programs as well, such as more functional strength, better injury prevention, greater ability to change muscle emphasis during exercises, greater muscle power development, and easier use.
One of the usually overlooked benefits of weights + resistance bands is that it prevents “cheating “ during the exercise. You see this often in gyms with free weights. Cheating involves using momentum to get the weight moving. Take the bench press for instance. A person can accelerate the downward or eccentric movement of the bench press to then build up momentum for the upward or concentric movement. Once the weight has built up momentum, less muscle fibers are activated to continue moving the weight throughout the rest of the range of motion of the exercise. Resistance bands do not allow the user to cheat by using momentum. This is because the resistance from the elastic band comes from the stretching of the elastic material and not the mass of the elastic equipment. The only way to continue a movement while performing an exercise with elastic resistance is to utilize more muscle fibers in the exercising muscle to continue stretching the elastic material.