The Ascension is a moment of turning, of change.
Jesus couldn’t stay around for always, not in that form. When he was appearing to them after the Resurrection, he was present with them in the place where the disciples were. He was preparing them for the coming of the Holy Spirit, who would guide them and encourage them and teach them, and the Holy Spirit would be with everyone at the same time, whereas the Risen Jesus could only be with the disciples at any one time. Jesus now goes to be with his heavenly Father, to take the throne as ruler of the world. He came from heaven to live on earth, and now he returns to heaven, because that’s how he can best go on loving the world.
And there is a change for the disciples. The Ascension marks the transition for them: before Jesus disappears, they are his followers, learning from him, observing him. Now they are separating from Jesus, becoming ready to become the church, Christ’s body on earth, and to lead the church to do the work that Jesus started.
When Jesus disappears, they are left looking up to heaven, looking for him. They have to stop looking at the place where Jesus was and turn to concentrate on the place where they are. Jesus isn’t going to come and rescue them, but he will give them the resources to get on with the business themselves, which is to tell people about God’s love and forgiveness, and live out God’s love in the places where they live and work and spend their time.