Why are quartz tubes rotated during the purification of molybdic oxide?

In the process of refining technical molybdic oxide into chemically pure MoO${}_3$ via sublimation, quartz tubes are inclined at a 20° angle and rotated to facilitate the continuous movement and uniform processing of the material within the electric retorts.1

This rotation, combined with the inclination, ensures that the charge is effectively exposed to the high temperatures (1,200 to 1,250 °C) required for sublimation. As the material is heated, the sublimed vapors are swept from the tubes by air to be collected in filter bags, allowing for the separation of volatile impurities from the final, high-purity product.1