Siman 237

It is the custom to say the verse: "And He, being merciful, will atone for iniquity" before 'Barchu', because in Shacharit and Mincha there are the daily offerings that atone, but this is not so in Arvit; therefore we request 'And He, being merciful, will atone for iniquity' (Tur), and there are additional reasons in the Tur and in the Kol Bo (see Beit Yosef). And there is a reason according to the Kabbalah, that there are three appointed ones in Gehenna: Mashchit, Af, and Cheimah, who judge the wicked at night because their deeds were in darkness, and for this we mention: "and He will not destroy (yashchit), and He frequently turns away His anger (apo), and He does not stir up all His wrath (chamato)"; therefore we do not say this on Shabbat, because on Shabbat the wicked have rest in Gehenna (Eliyah Rabbah in the name of Asarah Maamarot). "And He does not stir up all His wrath" means that He does not stir up any of His wrath, like "but the poor man had nothing (kol)" (ibid. in the name of our teacher, David Abudarham). However, Rashi, of blessed memory, explained that He does not stir up all His wrath, but He does stir up a portion of it, and so it is in the Yalkut; see there.

And some are accustomed to also say "A song, a psalm of ascents, behold, bless etc." before 'Barechu' or before 'Vehu Rachum' or afterward. And the reason is because in the Gemara they said: "If he is accustomed to read, he reads... and afterward he reads the Shema and prays," and they were accustomed to this psalm because it is written therein: "Who stand in the house of Hashem in the nights." And some add the verse: "By day Hashem will command His kindness, and in the night His song is with me," and it is of the same subject matter, obviously, and they also add the verse: "And the salvation of the righteous is from Hashem, their stronghold in a time of trouble," because the night is likened to troubles, therefore we supplicate Him, may He be blessed, that He should be our stronghold in a time of trouble. And they say the three verses: "Hashem of hosts etc.," because in the Yerushalmi, chapter Ein Omdin, it says that these verses should never move from your mouth, see there, and all our trust is in these verses, that He, may He be blessed, is with us and a refuge for us and He will answer us on the day we call. And they say half Kaddish, and specifically when there are ten who said these verses, and after the Kaddish the sheliach tzibbur says 'Barechu' etc.

The shaliach tzibbur does not repeat the prayer in the evening prayer because it is optional; so wrote the Rambam in chapter 9, law 9, and this is his wording: "And he does not return to pray aloud in the evening prayer, because the evening prayer is not an obligation. Therefore, he should not recite blessings in vain, for there is no person here who is obligated in them such that he would fulfill his obligation through him." Thus far his words. And even according to the Geonim who said that since one has prayed once it becomes like an obligation, nevertheless, whichever way you look at it: if he is proficient, the shaliach tzibbur does not fulfill the obligation for him, and if he is not proficient, behold, he has never prayed (Beit Yosef in the name of the Rashba). And even according to several of the Rishonim who wrote that we have all already accepted it upon ourselves as an obligation, nevertheless, regarding the repetition by the shaliach tzibbur we did not accept it upon ourselves; however, in truth, there are hidden reasons why there is no repetition by the shaliach tzibbur at night. And Kedushah is not said at night, and only the shaliach tzibbur says the full Kaddish, and we say Aleinu and the mourner's Kaddish and depart to our homes in peace (and there is no nefilat apayim in the evening prayer, and see the Tur, and with us it is not so as the Tur himself wrote, and examine this closely).