Sichos HaRan 239.
Translated by Rabbi Aryeh Kaplan, Rabbi Nachman’s Wisdom (Breslov Research Institute), pp. 374
The Rebbe said, “G-d’s ways are not like those of man. When a man makes a garment, he values it most when it is brand new. As it then becomes worn and damaged, its value is reduced. But G-d began by creating an imperfect world. It is constantly being improved and becomes more precious to G-d.
In each generation, Tzaddikim enhance G-d's work. At first there were the Patriarchs, Abraham, Isaac and Jacob, and Moses our teacher…. The world’s value to G-d is constantly enhanced by this. In the end, the Messiah will come and the task will be completed. Only then will the world be absolutely perfect.”
 Cf. Pesachim 119a; Bava Basra 88b; Sukkah 46b.