What makes resistance bands different from free weights?
Unlike free weights, resistance bands provide the exerciser with constant tension on the muscle - which is not possible with the use of dumbbells or barbells. This allows the exerciser to really focus on both the concentric (lifting portion) and eccentric (lowering portion) of the movement.
The working muscle actually shortens during the concentric phase of a movement and lengthens during the eccentric phase of a particular movement. Most lifters completely neglect the eccentric phase of a particular movement.
For example, while most lifters are performing a standard barbell curl, they often lift the weight up explosively and lower it back down at an incredibly fast rate.
Weight training is supposed to stimulate the muscle and recruit as much muscle fibers as possible. Don't expect much from lifting in that manner.
However, with the use of resistance bands, the muscle is provided with constant tension and the concentric phase and eccentric phase of the movement is much more controlled. This results in a completely new challenge for the muscles, which in turn forces the muscles to adapt.
Resistance bands will strengthen and elongate the muscle fibers. They also increase your flexibility tremendously. With the recommended short rest periods in between sets, expect your endurance and stamina to increase - which in turn will improve your cardiovascular health!