Why is the body of sin destroyed?
The body of sin is destroyed so that the old Adam, crucified with Christ, is put out of commission. Since the old man is killed and freed from sin, believers should no longer serve sin. A dead person cannot be tried or dealt with, illustrating that sin's hold is ended.
Les explains that we were dead in trespass and sins because of the old Adam we are born with, but have now been made alive. Coming back to verse 6, our old man, our old Adam, is crucified with Christ. Crucifixion kills, so God kills the old Adam and puts him out of commission. The purpose is that the body of sin might be destroyed or put out of commission, that henceforth we should not serve sin, for he that is dead is freed from sin. Les also uses the courtroom analogy: if someone dies before the case is turned over to the jury, it is all done, the case is ended and closed, because you cannot try a dead man or do business with a dead person. Likewise, if our old Adam is dead, we can no longer deal with him; that is the whole idea.
Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.