To put everyone in the world to sleep in order to create Utopia
The Utopia Spell is a non-verbal spell created by the Avatars to create Utopia, their perfect world. In order to successfully cast it, they required the power of the Charmed Ones. The spell put everyone to sleep for a few hours while their minds were cleared of the duality of Good and Evil, allowing them to accept Utopia.
When the sisters agreed to create Utopia, the Avatars began preparing the spell. Kyle Brody, who was working with Zankou, tried to stop the process by infecting the sisters with a Paranoia Crystal. However, the sisters went through the spell despite their doubts and the world was put to sleep.
While humanity slept, the sisters cleared out the remaining demons. When Kyle and Beta killed each other, the other Avatars stepped in and put the sisters to sleep, so these events would not disrupt the creation of Utopia.