Matthew 18:16

The Mosaic Law had also advocated the second step that Jesus taught (Deuteronomy 19:15). However, Jesus broadened the field of civil law that the Deuteronomy passage covered to include any sin about which a disciple might need rebuke. Jesus was not perpetuating the whole Mosaic Law. He was simply carrying over these provisions in the Law that He declared were now binding on His disciples.

Probably the function of the witnesses is to witness to the erring disciple’s reaction to the confrontation. This seems to have been the purpose in the Deuteronomy passage. Their presence would be an added inducement to return to the fold of the faithful. These seem to be witnesses to the confrontation, not to the sin. If the brother or sister proved unrepentant and the initiator needed to take the third step (Matthew 18:17), witnesses to the confrontation might be necessary.