Descending the worlds after Shemoneh Esrei:
In Kabbala, it is explained that the Shacharis prayer is likened to a ladder, in which we climb up the four worlds of Assiya, Yetzira, Beriya, and Atzilus. The summit of Atzilus is reached during Shemoneh Esrei. Now, just as we climb the worlds during the Davening, so too, we descend back below, so we return back to earth. When does this descent take place? Starting from Tachanun. While Shemoneh Esrei is the ascent to Atzilus, Tachanun is the descent from Atzilus. Ashreiy and Uva Letziyon, in which we mention the angels, is similar to Birchas Shema and is the descent from the world of Atzilus into the world of Beriya. Shir Shel Yom is similar to Pesukei Dezimra, in which we sing praise to G-d, and represents the descent from the world of Beriyah to the world of Yetzira. Ein Kelokeinu and Pitum Haketores is similar to Hodu, in which we give thanks to Hashem, and represents the descent from the world of Yetzira to the world of Assiya. Aleinu Leshabeiach represents the landing back on earth.
 See Derech Mitzvosecha Shoresh Mitzvas Hatefila 11 [p. 120]; “Ladat Eich Lihitpalel” [Gopin] volume 4 Hakdama