The bimah is placed in the center of the shul to enable everyone to clearly hear krias haTorah (the Torah reading). Another reason for placing the bimah there is because it represents the Mizbeach Hapnimi (the altar in the inner sanctuary of the Holy Temple) which was positioned in the center of the sanctuary between the Menorah (Candelablra) and the Shulchan (the table on which the special loaves of bread were placed). It was on this altaralso called the Mizbeach Hazahav (the golden altar)upon which the Ketores (Incense) was offered. On Succos we circle the bimah holding our lulavim and esrogim reminiscent of the service that was conducted around the Mizbeach in the Beis Hamikdash. However, the main reason for placing the bimah in the center of the shul is because the Reform synagogues place the bimah next to the Aron Hakodesh (the Torah Ark); since it is forbidden to emulate the ways of those who do not adhere to Torah law, we place the bimah davka (specifically) in the center of the shul. Nevertheless, it doesnt have to be positioned exactly in the center, but may be placed slightly to either side. At the very least there should be one row of seats between the Aron Hakodesh and the bimah in order for it to be considered being in the center of the shul. In a beis hamidrash, a place designated for Torah study, and not for davening (prayer), there is more room for leniency regarding the placement of the bimah.