Why did Judas need to identify Jesus?

They perhaps thought that the one betraying Christ to them is more trustworthy than the word of any of those they sought, thinking them to be criminals (who would, according to them, presumably be capable of concealing who was their Master).

This was also perhaps a common way to greet their Rabbi, a kind of faire la bise along with "Hail." They perhaps told him to greet Him as they would normally, so that nothing seemed out of the ordinary, or so that he was not suspected of anything when appearing out of nowhere in the Garden they were probably so accustomed to meet in.

I have heard from several old parishioners at my Roman Catholic church that St. James (the greater) was so visually similar to his cousin Jesus that only people very close to them could tell the difference between the two, and thus the kiss of Judas was necessary to identify the correct man. I can't find any corroborating sources online, but I've heard this from a fair number of people and that St. James was a disciple and related to Jesus is certainly a supported belief.

Keep in mind that this event coincided with the Passover. There would have been many people in Jerusalem. Finding one person was not an easy task. There also would have been many rabbis, each with dozens of followers/disciples. Even if the Jewish leaders what Jesus looked like, and where he was (and how could they know his precise location without some inside help), the Roman soldiers who went to arrest Jesus might not have known. As DJClayworth pointed out above, Jesus had more than 12 disciples, so they (the soldiers) probably needed someone to point him out to them.

Here are some things to help clarify this situation.

First, "his disciples" does not always refer to the Twelve. It can refer to a much larger number, including "the seventy" who Jesus sends out to spread the message. Matthew in particular usually refers to "The Twelve" or "the Apostles" when he is talking about the smaller group and "the disciples" can mean a larger group.

Second, even if the Jewish leaders knew what Jesus looked like (not at all certain - how would they know that? Would they have gone to hear him speak? How would that have looked to the crowd?) then it's far from certain that the people they sent to arrest him did. Many of them might have been temple guards, or just hired thugs. No reason why they would ever have seen him before.

Third, don't underestimate the 'bad light' argument. The torches would not have been modern flashlights, but flickering flames.

Fourth, don't underestimate the symbolism. Maybe God required Judas to betray with a kiss in order to bring home the magnitude of the betrayal.

Fifth (and this is my own speculation) the priests may have wanted Judas present to make sure this wasn't some trick Judas was playing to get the wrong man, or to get his money without actually betraying Jesus. Since Judas would have been needed to get them to the exact place, it makes sense for him to give the identification.

I think it's necessary to separate this question into two parts:

1. Why did Judas need to lead the mob to Jesus? and
2. Why did Judas need to kiss Jesus to identify him?

I will attempt to answer the first question. Some possible explanations for the second question can be found here: https://hermeneutics.stackexchange.com/questions/16864/why-did-judas-betray-jesus-with-a-kiss

So, why did Judas need to lead the mob to Jesus?

Jesus and the 12 often retreated to Gethsemane at the Mount of Olives (presumably for solitude, prayer, and rest), as indicated in Luke 22:39, "Jesus went out as usual to the Mount of Olives, and his disciples followed him."

Having just shared the Passover feast with Jesus and the rest of the 12, Judas certainly would have been made privy to where Jesus was going after the meal: a place that Judas knew well from his previous visits with Jesus and the rest of the 12 (John 18:2).

We know from Luke 22:6 that Judas "...watched for an opportunity to hand Jesus over to them when no crowd was present." I would conjecture that Judas found this to be the best opportunity to arrest Jesus because of a number of favorable conditions:

1. Jesus was (mostly) alone. Jesus was in a secluded place outside of the city with only his 11 closest disciples. No crowds of listeners. Should there be any resistance, the cohort that came to arrest Jesus would have no problem overpowering the 11 disciples.
2. It was late. According to chapter 9 of the Pesachim in the Babylonian Talmud, it was Jewish tradition to spend the night in Jerusalem during the Passover (http://juchre.org/talmud/pesachim/pesachim4.htm#95b). Therefore, it is safe to make the assumption that those celebrating the Passover turned in for the night after the feast. It would have been highly unlikely for there to have been any Jewish citizens and travelers outside. This was particularly important because other followers of Jesus would not find out about the arrest, so there was no reason to worry about retaliation. Remember, those seeking to kill Jesus feared his large following and saw the crowds as a real threat (Mark 12:12).
3. It was dark. The enemies of Jesus were most concerned with transporting him to the Sanhedrin as quickly as possible where he could be tried and charged for blasphemy (a capital crime) in the middle of the night. The darkness provided enough stealth to capture Jesus without drawing the attention of onlookers. The goal was to create as little commotion as possible, and the darkness helped hide what was happening.

Mishnah Sanhedrin 4.1 says that capital cases (that is, crimes punishable by the death penalty) cannot be tried at night (http://www.sefaria.org/Mishnah_Sanhedrin.4.1-5?lang=en&layout=lines&sidebarLang=all). The accusers of Jesus knew the law, so trying him at night ensured that they could control the trial (witnesses and verdict) without interference from others who may have otherwise defended the innocence of Jesus.

Judas recognized that the conditions were perfect for Jesus to be arrested, transported, and tried. I would guess that after the Passover meal, Judas went to the Chief Priests and convinced them that it was an ideal time to arrest Jesus. Of course, only Judas knew exactly where Jesus was in Gethsemane, so he naturally had to be the one to lead the cohort. Judas was the key the Chief Priests needed to arrest Jesus.