BCAAs include leucine, isoleucine, and valine, and they are among the nine essential amino acids that the body is unable to make on its own. They support everything from anabolic muscle building to high-intensity endurance training to improving mental function and mood.
The amount of good that having enough BCAAs in your diet is hard to under estimate. There are pages and pages of studies on each of these points but I am just going to give an overview so you can study this further on your own: they improve insulin sensitivity, decrease soreness, increase strength gains and brain function, minimize muscle loss and the cortisol response that comes from the stress of exercise, protect the liver, increase the formation of new mitochondria, help weight loss, etc.
Conclusion: you should be supplementing with BCAAs before, during, and after workouts.