Page "Common drain" Paragraph 1

In addition, this circuit is used to transform impedances.

For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower, a small resistance.

That resistance reduction makes the combination a more ideal voltage source.

Conversely, a voltage follower inserted between a driving stage and a high load ( i. e. a low resistance ) presents an infinite resistance ( low load ) to the driving stage, an advantage in coupling a voltage signal to a large load.