At its core, resistance bands provide tension across your exercise's entire range of motion. That tension is what challenges muscles, but they do so differently than free weights.
In general, Tonal coaches explain that using resistance bands can be a solid choice to support muscle engagement and practice control and consistency.
However, strength progress and muscle-mass development will come from gradually increasing load. With resistance band exercises, you can increase load through things like performing more reps and sets, decreasing rest times, improving range of motion, and slowing down movements. But after months of this, you may hit a plateau in the total loading that bands afford your body. At that stage, it may be time to try free weights.