Tefillah Tips - Barchu
There are three times in our prayers when the Barchu is recited. The 1st is after Yishtabach preceding the blessings before Shema in the morning, the 2nd before each Aliyah read from the Torah and the 3rd at Maariv, the evening prayer introducing the blessings before Shema as in the morning. Unlike the Shema and personal Amidah, the Barchu before reading the Torah may only be recited in the presence of a Minyan/quorum.
Rav Schwab zt"l explains that the Barchu format is an invitation. The leader/reader/Chazzan calls to the entire congregation and invites them to say Barchu with him as if to say: I may be the leader, the Torah reader, the Chazzan but A) this is not about my prayer but about all of our prayers and B) what we can accomplish together clearly surpasses any individual prayer uttered in the world. Therefore-join me - Barchu et Hashem Hamevorach - Together let us acknowledge the source of all blessing.
The Talmud explains that this interchange is based on the verse Ki shem Hashem Ekra Havu godel Laylokeynu - which means - To the name of G-d I will call, give greatness to our G-d. Indeed when the congregation responds, the result is stronger and more powerful as is indicated in the response, Baruch Hashem Hamevorach Leolam Vaed - Let us acknowledge the source of all blessing for eternity. The sum total is always greater than the individual parts.
Our sages also teach that the recital of Barchu by a mourner provides comfort and protection to the deceased. It is because of this that a mourner has certain rights and privileges to lead the davening.
Barchu is a wake up call to all those participating in the prayers to join forces and pray together as one. By saying the Shema and reading the Torah we are demonstrating that the G-d of Israel is One. Therefore by reciting the Barchu together, we unify ourselves as one in order to properly declare the sovereignty, omnipotence of Hashem, the one G-d of the heavens and the earth.