When Lodge is at labor, brothers do not pass between the altar and the East – why is that?

Brethren do not pass between the Altar and the East in a Masonic lodge at labor (except in a degree) because the Master is supposed to have the Great Lights constantly in view. In theory, at least, he draws inspiration from the Altar to preside over the lodge and must not, therefore, be prevented from seeing it at any time.
The custom is but a pretty courtesy, but it is rooted in a fundamental conception of the Craft— that the Altar is the center of Masonry, and that from it and from the Great Lights it bears, flow and all there is of Masonic inspiration and truth and light.
English lodges do not have this problem, since in them a pedestal near the Master is the Altar in which lies the Holy Book.