Question
Why do we bow, specifically during the “barchu” prayer?
Answer
The Mishna Brura (in his Be’ur Halacha, 113 “”hakoraya.”) writes that the source is the verse in Divrei Hayamim (Chron.) 1, 29; 20 where it says “And (King) David said to all the congregation, ‘bless, please, the Lord your God ‘ and the whole congregation blessed the Lord, the God of their fathers, and they bowed and prostrated themselves to God.”
From here it would seem that blessing God should be accompanied by some physical action demonstrating (at very least for ourselves) our readiness to humbly submit our lives, hearts and minds to His service.
Rabbi Boruch Clinton