Short ideas from the Zohar, the Chassidic masters and Rav Kook.
Zohar
This part is for the world
The Zohar, the central book of Kabbalah, says prayer repairs four things, in order: yourself, this world, the world above, and God's name. The psalms that start here are the second: you bless God for each piece of creation, to keep this world standing. And so, it says, Baruch She'amar goes "blessed, blessed": blessed over everything.
וְעַל דָּא בְּבָרוּך שֶׁאָמַר, בָּרוּךְ, בָּרוּךְ עַל כֹּלָּא
Chassidut
Says, not said
Rabbi Chaim of Chernowitz (Ukraine, about 1800), known by his book Be'er Mayim Chaim, writes that God gives life to all of creation at every moment. If that stopped for even one moment, the world would be as if it had never been. That is why we say "says and does," not "said and did": at every moment He is saying, and doing.
ולזה אנו אומרים ברוך אומר ועושה ולא אמר ועשה, כי בכל רגע ורגע הוא אומר ועושה
Rav Kook
Why borrow King David's words?
We praise God "with the songs of David Your servant." Why his words and not our own? Rav Kook (the first Ashkenazi Chief Rabbi of the Land of Israel, died 1935) answers in his commentary on the siddur, Olat Re'iyah: our private feelings are not enough. We draw on the pure feeling stored in the treasury David left us, because David had the soul of the whole nation.
שהיא גנוזה באוצר הקדש שהניח לנו דוד מלך ישראל, בעל הנפש הכללית של האומה כולה
Rav Kook
Why a tune?
No words can match God's greatness. Tehillim says that to God, silence is praise. So spoken praise has to quiet the feeling down to fit it into words. But a feeling from the heart goes past the narrow circle of words and ideas. That, Rav Kook says, is the power of melody, which goes along with praise. The Tur, too, says to sing this blessing.
וזהו כח הזמר, המלוה את השבח
Rav Kook
Living and lasting
"He lives forever and lasts for all time." In everything we know, those two do not go together. Life means changing all the time, and something that never changes looks to us like something that is not alive. Only about God, Rav Kook says, can both be said at once: fully alive, and never changing.
תאר חי וקים ביחד, במלא מובנו, לא יש שום אפשרות לציר אצל שום נושא
Chassidut
Ten sayings, Ten Commandments
The Sefat Emet reads "says and does" as the ten sayings of creation, and "decrees and carries it out" as the Ten Commandments. The Torah is what sets the whole of creation right.
פרשנו אומר ועושה הוא עשרה מאמרות. גוזר ומקיים עשרת הדיברות. פי' שהתורה היא תיקון כל מעשה בראשית