So why did he write this letter?

John wrote his letter to protect his readers but there were other reasons as well. He wrote to produce joy, holiness and assurance among his readers. Joy cannot be artificially produced but it comes as a byproduct of our service for the Lord. Phillip Yancey says that he has interviewed many stars and servants but it is the servants that have the joy. Holiness had been under attack by the teachers of John's day. Since the body wasn't important then why worry about holiness? Finally, his readers felt that they had come short because they did not have the experience of the superchristians of his day. John reminds his readers that our assurance is not based upon our experience or our works but upon the finished work of Christ.