The appropriate sequence of prayer is to begin by praising Hashem and only afterwards to approach Him with our requests. We learn this from Moshe Rabbeinu who commenced his prayer with words of praise, saying (Deuteronomy 3:24), “Hashem, God, You have begun to show Your servant Your greatness and Your mighty hand. What force is there in heaven or earth that can perform deeds and mighty acts as You do?” Only afterwards did Moshe plead, “Please let me cross [the Jordan] so that I may see the good land…” Based on this, Rav Simlai interprets, “A person should always praise Hashem first and afterwards pray” (Berachot 32a). This is the order of the berachot in the Amidah. The first three berachot open with words of praise and only later continue to the berachot detailing our requests. Even in the rest of the prayer service it is proper to begin in this fashion; therefore the Chachamim established the recital of Pesukei d’Zimrah, words of praise and exaltation of Hashem. The Chachamim instituted opening Pesukei d’Zimrah with Birkat Baruch She’amar and ending it with Birkat Yishtabach.
During the time of the Tanna’im, the recital of Pesukei d’Zimrah was considered to be an extra pious act praised by the Chachamim. As time went on, however, the minhag spread until it became an obligatory custom, and now all of Israel commence their prayers with Pesukei d’Zimrah.
By reciting Pesukei d’Zimrah, a person reflects upon the magnitude of the Creator, and thus when he subsequently prays, he knows before Whom he stands. Otherwise, he might come to request his needs like idol worshipers, whose whole aim is to achieve personal success in their lowly matters, and who are not interested at all in devoting themselves to Hashem, the Source of Life. However, a person who purifies his heart by meditating on the greatness of Hashem knows how to pray. Even when requesting health and livelihood, he does so in order to devote himself to Hashem’s Torah and to rectify the world under the Almighty’s sovereignty. In this spirit, his prayers will be accepted (see Olat Ra’aya part 1, p.14).
The name “Pesukei d’Zimrah” means “verses of songs of praise.” Additionally, it derives from the phrase “zemirat hakerem” (the pruning of a vineyard). Just like one who prunes his vineyard cuts off the extra branches in order to increase the growth of the vines and thus produce better fruit in the future – by saying Pesukei d’Zimrah we destroy our flawed thoughts and negative feelings, so that our prayers will ascend upward and be accepted. This process of mental and emotional purification inspires us to joyous song; therefore these passages are called “Pesukei d’Zimrah.”
.Berachot 4b states, “Whoever recites Tehillah L’David (Ashrei) three times daily is promised life in the World to Come,” and Shabbat 118b adds, “Rabbi Yossi says, ‘May my portion be among those who complete the Hallel (meaning Pesukei d’Zimrah) every day.’” From these words it can be inferred that the Chachamim did not institute the recital of Pesukei d’Zimrah; rather, it is an extra pious act that all of Israel practiced, and consequently became an obligatory custom. As the Rasag writes in his siddur (Mekitzei Nirdamim publishers, p. 32), “Our nation has volunteered to recite a number of Psalms praising HaKadosh Baruch Hu – with two berachot before and after them…” Additionally, in the words of the Rambam (Tefillah 7:12), “The Chachamim praise a person who recites songs from Psalms each and every day, from Tehillah L’David until the end of the book. It is already customary to recite verses [of Scripture] before and after them, and a berachah was instituted for recital before the Psalms … and a berachah after them.” The wording of these berachot is first mentioned in Seder Rav Amram Gaon, and some conclude that these berachot were instituted during the period of the Geonim. (There are even poskim who maintain that, halachically, Baruch She’amar and Yishtabach are less important than the rest of the berachot, as brought by the Bei’ur Halachah 51:2 s.v. “Im”). However, Tola’at Yaakov writes in the name of Or Zarua that Anshei Knesset HaGedolah instituted the wording of Baruch She’amar from a note that fell from Heaven. Many Acharonim write this, too, as brought by the Mishnah Berurah 51:1. The Mishkenot Yaakov writes that the berachot were composed during the period of the Tanna’im and therefore it is forbidden to skip them. Some maintain that Rabbi Yishmael instituted them, as written in Sefer Likutei Maharich (see Makor HaTefillot p. 13). Although Rabbi Simlai learns (in Berachot 32a) from Moshe Rabbeinu’s prayer that a person must begin praying with words of praise, Rashi in Avodah Zara 7b interprets this to mean that we must begin the Shemoneh Esrei with three initial berachot of praise. So writes the Rambam chapter 1, halachot 2 and 4. If so, there is no biblical basis for Pesukei d’Zimrah, rather its recital is an extra pious act.
However, it is implied from the Rif (Berachot 23a), Rosh (Berachot, chapter 5, 4-6) and Tur that Pesukei d’Zimrah were instituted in order to start the prayer service with words of praise. The Bach, Orach Chaim 51 explains that the Chachamim instituted berachot for Pesukei d’Zimrah based on Rabbi Simlai’s extrapolation from the verses. He reasons that although they did not obligate the recital of Pesukei d’Zimrah, if there was no certain teaching from the Torah that this was the order of prayer, they would not have instituted the recital of berachot for them.
See note 9 further in this chapter which states that according to Rav Natrunai and the Rashba, if one did not recite Pesukei d’Zimrah before the Amidah, he may not recite them after it, for their whole purpose is to prepare a person for prayer. However, according to Talmidei Rabbeinu Yonah and the Rosh, one may make them up after the Amidah, including the berachot. It must be explained in their opinion, that even though they were instituted before the Amidah, nevertheless, b’dieved, they still possess value even after the Amidah, for their main purpose is to praise Hashem and not necessarily to serve as preparation for prayer.