The increase in force production while using bands appears to lead to better strength gains than just using free weights alone when following a training program, especially for well-trained individuals.
How should you implement this? Start with about 60% of your 1RM, or a weight you can do for 10-12 reps. Add a band on each side and still try to hit 10-12 reps!
If you want to increase power production, start with about 40% of your 1RM on the bar, add a band on each side, and perform sets of 1-4 reps while moving the bar as fast as possible.
Source: Ataee, J., Koozehchian, M. S., Kreider, R. B., & Zuo, L. (2014). Effectiveness of accommodation and constant resistance training on maximal strength and power in trained athletes. PeerJ, 2, e441.