Is the custom of situating the bimah on the eastern wall of a synagogue a custom or merely a custom or is it a requirement?
Interestingly, there are actually two components to a synagogue which serve as the center of attention, so to speak: the Bimah (which literally means a platform) and the Amud (literally, a lectern, or a place to stand). Each component serves a separate purpose. The Bimah serves as the location for the reading of the Torah, while the Amud serves as the location where the Chazan (the one who leads the prayers) stands.
Each of these two components has a different recommended location. The Talmud mentions that the Amud should be situated close to the Ark where the Torah scrolls are kept, so that the Chazan, as he leads the congregation in prayer, can gain inspiration and be awed by his proximity to the holy Torah scrolls. Since in most synagogues, the Ark is placed on the Eastern wall, so as to face Jerusalem (at least in the Western Hemisphere; in other locales, the Ark is usually situated toward Jerusalem, whatever direction that may be), the Amud would also be situated close to the eastern wall.
The recommended practice for the Bimah, on the other hand, is to situate it in the middle of the synagogues so that everyone in the sanctuary will be able to clearly hear the reading of the Torah. If for some reason, the placement of the Bimah in a different location (other than the middle of the sanctuary) will afford more people the opportunity to hear the reading of the Torah, the Bimah may certainly be placed in that alternative location.
As I hope I have implied, however, the above guidelines are exactly that: guidelines and recommendations based upon the reasons I have provided above. Certainly, these guidelines do not carry the severity of a commandment or law.
I hope this helps clarify the issue for you.
Be well,
Rabbi Yoel Spotts