Question: On Friday night the chazzan in many shuls ascends the bimah for Kabbalat Shabbos but goes to the amud starting for Barchu. Why?
Answer: HaRav Shmuel David Munk suggests that the rationale for the chazzan to lead Kabbalat Shabbos from the bimah instead of his usual place at the amud is to demonstrate that reciting the mizmorim of Kabbalat Shabbat is not an obligation but merely a custom (Kuntress Torat Imecha – Synagogue Customs, #39, footnote 33). (This is also why the singing of Yigdal or Adon Olam also often takes place from the bimah.)
Another reason why the shliach tzibbur goes to the amud – which is right in front of the Aron Kodesh – for Barchu is to dramatically note that the onset of Shabbat brings Jews spiritually closer to the ideals of the Torah contained within the Aron. (This rationale was gleaned from a discussion with my late father-in-law, HaRav HaGaon Rav Yaakov Nayman, zt”l, a noted disciple of the Brisker Rav.)
In general, the place of important synagogue activities has spiritual overtones. A prime example is the bimah. When someone is called to the Torah, he has an aliyah, which means “going up,” since he must ascend the elevated bimah. In contradistinction is the amud. Whenever the Talmud discusses leading services, it uses the term “yored – going down,” indicating that the shliach tzibbur used to stand in an area that was lower than the congregants of the synagogue. (This was based on Tehillim 130: “From the depths I call to you G-d.”) This lower area was meant to impress upon the chazzan the need for humility and to instill religious concerns in him.