Festival Prayers-Succot Kavanot-Ha’Aderet v’ha’Emunah
The Sefer haRokeiach teaches that Psalm 136, the Hallel haGadol, the Great Hallel, is actually sung by the Highest Angels, and he lists the names of the Angels that sing each line, with the Sefirah represented. We do not practice chanting the names of those angels, so we recite instead this song which is another of the Great Songs of the Angels.
One who spends his time in his sukkah in sanctity and with focus, merits that the Angels join him and sing with him. We sing this song on Sukkot as an invitation to the Angels to join us in Songs of Praise in the Sukkah.