This is to indicate the beginning and end of the trance.
And you notice that the beginning of the trance of the Mass is the Intro, or introductory procession. The Intro is to induct the trance.
And the trance of the Mass ends with the words, "Ita missa est", (the mass is ended). This is to signal the end of the trance and wake up the congregation so they can take what they have learnt out into the world, wide awake.
So first I put you to sleep, then I wake you up.
In the meantime, we commune together in communion.
This is our sacrament that re-enchants the world.
And all we need to do to re-enchant the world is to chant together.