The purpose was to give the TV audience a greater sense of actually being in space. It didn't make a lot of practical sense, but then again having the bridge module where it was -- right on top of the saucer -- never made much sense to me either. Way too vulnerable there. It should have been hidden right in the middle of the saucer where it would be better protected and allow for easier and more direct access to other parts of the ship in an emergency situation. But sometimes practical considerations have to be weighed against the dramatic needs of television series.