"'What are you arguing with them about?' he asked." NIV translation
Have you ever wonder why Jesus asked questions where He already knew the answer? The simple reason is that He did so for our benefit as we see here. If He had just rushed in and fixed the problem, we would be missing out on important teaching.
"A man in the crowd answered, 'Teacher, I brought you my son, who is possessed by a spirit that has robbed him of speech. Whenever it seizes him, it throws him to the ground. He foams at the mouth, gnashes his teeth and becomes rigid. I asked your disciples to drive out the spirit, but they could not.'" NIV translation
A man in the crowd tells the Lord that he brought his son to be cured of demon possession. He went on to explain that the disciples had tried but were unable to make the demon spirit leave.
"'You unbelieving generation,' Jesus replied, 'how long shall I stay with you? How long shall I put up with you? Bring the boy to me.'" NIV translation
You can almost feel the disappointment that Jesus felt with His disciples. He had told them as well as showing them the power that comes through faith in Him and yet, when put to a test, they failed.
As Christians, we sometimes forget how big a sacrifice it would have been for Jesus to leave His Father's side in heaven and come to this earth. It is just as disappointing to see people today that say they have faith in Jesus Christ but do not walk in His power.
"So they brought him. When the spirit saw Jesus, it immediately threw the boy into a convulsion. He fell to the ground and rolled around, foaming at the mouth." NIV translation
We see the demon react to the presence of Jesus and are reminded that the devil and his demons know who Jesus is and they tremble at His name.
"Jesus asked the boy's father, 'How long has he been like this?' 'From childhood,' he answered. 'It has often thrown him into the fire or water to kill him. But if you can do anything, take pity on us and help us.'" NIV translation
Why are children (or anyone else) afflicted with illness or other stuff like this? As we see here, the demon had often tried to kill the child. The survival of this child and his being presented to Jesus was so that others could see the power of God and believe.