The disciples must have wondered how Jesus going away could be better than having Him here, face to face. But Jesus knew that His time to leave was very near at this time. He also knew that the Helper could do something that He, in human flesh, could never do. That was to actually live inside a human being. The Helper, the Holy Spirit would take up a permanent residence inside those that accepted Jesus as their Savior. The Holy Spirit could be omnipresent, He could dwell in all of us at the same time thus as God had promised, we would never be alone, He would always be with each of us. He will prove to unbelievers that not believing in Jesus is the sin that caused Jesus to die on the Cross. He will prove that judgment happened when satan was judged. This judgment is spoken of in Isaiah.
12 King of Babylon, morning star, you have fallen from heaven, even though you were as bright as the rising sun! In the past all the nations on earth bowed down before you, but now you have been cut down–Isaiah 14:12 (NCV)
This judgment was also the event Jesus was referring to in Luke 10, when He said:
18 “Yes,” he told them, “I saw Satan fall from heaven like lightning! 19 Look, I have given you authority over all the power of the enemy, and you can walk among snakes and scorpions and crush them. Nothing will injure you.–Luke 10:18,19 NCV)
In this statement, Jesus tells us He was there in heaven when satan was judged and He witnessed satan being cast out of heaven. And now, Jesus has the blessings of the Father to give the disciples and us authority over all the power of the enemy. The Holy Spirit is the guarantor of that promise of authority and He fights our battles for us.