These relube flanges are used where additional lubrication is required. For example, where dust, dirt, excessive moisture, or other contaminants are present. The additional lubrication increases the life of the ball bearing unit by flushing out any contaminants that may be present, plus lubricating the ball bearing unit over a longer period of time.
The flange with the grease fitting has an inner grease groove, which allows the grease to enter the holes in the outer ring of the ball bearing. The other mating flange has a similar grease groove, so the ball bearing can be reversed, and still be lubricated.