The apostles at the last supper did not know who was the betrayer among them even after Jesus identified to them it was Judas by the dipping of bread

John 13:27-30 Then Jesus said to him, "What you do, do quickly." 28 But no one at the table knew for what reason He said this to him. 29 For some thought, because Judas had the money box, that Jesus had said to him, "Buy those things we need for the feast," or that he should give something to the poor. 30 Having received the piece of bread, he then went out immediately. And it was night.”

So the apostles needed to know without any dispute that it was Judas

Matthew 26:48 His betrayer had given them a sign, saying, "Whomever I kiss, He is the One; seize Him."

The Scripture tells us Acts 1:16-17 He was a guide to those who arrested Jesus; Judas went with a detachment of troops, and officers from the chief priests and Pharisees (John 18:3).

First - it was dark and the torches and lanterns they used would not light up that much area. Second- Jesus looked like any other Jewish man so the Romans needed Judas to identify him to be sure. What makes this more Interesting is that Jesus identified himself to his arresters. John 18:4-5 he went forward and said to them, "Whom are you seeking?" 5 They answered Him, "Jesus of Nazareth." Jesus said to them, "I am” V:7 when--when He said to them, "I am,"--they drew back and fell to the ground.” He identifies who he really is by the use of the divine name given to Moses. Jesus then makes sure his disciples are able to get away