Cloning is about duplicating content (e.g., two windows sharing the same content) by sharing buffers.
Cloning can be used to improve performance:
The number of updates required can be reduced.
You can have content shown on multiiple displays, but only have to update one buffer. Video playback is a good example. You can decode once, but playback on multiple displays.
The amount of blending in the display can be reduced.
For example, consider a window that's entirely transparent except for a watermark that needs to be blended in one corner. Blending this entire window is costly and can be avoided altogether by setting the transparency of this window to SCREEN_TRANSPARENCY_DISCARD. However, in order to keep the watermark visible, you can create another window that shares its buffers with the transparent window. This way, the window that's transparent is mostly discarded and only a small area, the size of the watermark, is actually blended.