The demon did him all possible harm in leaving him. Indoors the nine disciples seek an explanation for their colossal failure. They had cast out demons and wrought cures before. It is clearly a late addition to help explain the failure. But it is needless and also untrue.
Prayer is what the nine had failed to use.
They were powerless because they were prayerless. Their self-complacency spelled defeat. That is true also. They had too much faith in themselves, too little in Christ. He was not willing that any one should learn it.