Two possible reasons that Jesus called his disciples before preaching to the crowds:
Jesus is already demonstrating his ability to shine light into the hearts of those whom he makes his disciples, showing that he is indeed the Light prophesied by Isaiah.
Jesus is calling them to witness to his word and work (John 15:26–27; Acts 1:21–22), to later tell what they have seen and heard (Acts 1:8; 1 John 1:1–5) by the Spirit’s enabling (John 14:26, John 16:13).
Interpretation 1:
It would be an error to downgrade the calling of the first four disciples recorded in Matthew 4:18–22 to a mere biographical note. It is much more than that, for it is a promise given at the commencement of a great future and one which manifests in Jesus’ actions (Jesus can command and turn men into disciples). This shows how the people who dwell in darkness will get to see the Light because it is Jesus that opens their eyes, bends their will, and commands them to follow him as King.1
Interpretation 2:
Before, the preaching of Jesus acquired a different character to that of John the Baptist, he called the first four of the apostles to follow him. They were called to testify of his word and his work from the very beginning (John 15:26–27; Acts 1:21–22) by proclaiming what they have seen and heard (Acts 1:8; 1 John 1:1–5). They would be equipped to do this by the special grace of the Holy Spirit (John 14:26,John 16:13). For this reason the apostles already had to be called right at the start of Jesus’ ministry. Their presence was required.2
18 While walking by the Sea of Galilee, he saw two brothers, Simon (who is called Peter) and Andrew his brother, casting a net into the sea, for they were fishermen.