Did John send two of his disciples to Jesus for his own sake or for the sake of his disciples?
John was filled with the holy spirit from his mother's womb. Miraculously, he identified Jesus as Lord and Messiah even before either was born. (Luke 1:15, 41-45) At Jesus' baptism, John saw the holy spirit descend and identify Jesus. He also received a supernatural revelation when he heard God's own voice declare that Jesus is the beloved Son of God. (Matthew 3:17) John had pointed others to Jesus as the Messiah many times and had given frequent testimonies that he was the Lamb of God and bridegroom of his church. "I have seen and have borne witness that this is the Son of God," he said. (John 1:34) So it seems highly unlikely that John sent the disciples to Jesus for the sake of his own belief and faith.
John was in prison and his disciples may have begun to have doubts about his ministry and message. Jesus' own disciples experienced similar difficulty when Jesus finally got through to them that he himself was going to be executed. John apparently wanted his disciples to be fully persuaded by experiencing firsthand that what he had been saying about Jesus was totally true.
It is incomprehensible that John himself was experiencing doubts. If he was, then Jesus' praise of him after the disciples left would have been meaningless. He spoke highly of John as a powerful man of God, telling the people that there was no one greater and that John was not a weak person who could be easily blown about by changing circumstances. How pathetic a figure John would have been if, after pointing thousands to Christ, he eventually entertained doubts about the message concerning Christ that God had sent him to preach! Such doubts would surely have been an embarrassment to the multitudes led by John to repentance and baptism, and they would have been an embarrassment to Jesus himself.
Being full of the holy spirit, John possibly knew that he was soon going to be executed. He had led others to Christ, but some men had chosen instead to remain behind as John's own disciples. John had put forth an earlier effort to persuade them that Jesus and not he himself was the Messiah. (John 3:25-36) They did not see themselves as disciples of Jesus but preferred to be known as belonging to John. (Matthew 9:14) John wanted them to attach themselves to Christ rather than be left bewildered and disappointed when John was no longer around to spiritually lead and sustain them. Interestingly, after John had been beheaded, "his disciples came and took away the body and buried it; and they went and reported to Jesus." (Matthew 14:12) The very fact that these disciples, now that their master had been murdered, reported this to Jesus would seem to indicate that they finally believed in him fully.