You perform a burnout set at the end of a body part workout to completely fatigue the muscles. A burnout set involves lighter weights and very high repetitions -- from 15 to up to 100, to encourage maximal glycogen depletion. This training protocol can be beneficial for muscle growth. But if you use burnout sets all the time, you could run the risk of overtraining, developing injuries and slowing your gains.
A Better Option
If you want to do burnout sets occasionally, that's fine -- but it shouldn't be a normal part of your regimen. If your goal is muscle growth, use enough resistance so the final repetitions of your eight- to 12-rep sets are difficult to complete, but do not go to the point of failure. Stick with proper form, regardless of how much weight you use. Vary your training so as to incorporate high-intensity, heavy training; light, high-rep workouts; and most of the time, moderate resistance workouts.