If you're running your RO unit on relatively clean water, this feature is probably not needed. However, if the TDS of the water you're treating is high, this feature will be more than worth the expense.
The purpose of the auto-flush feature is to flush the membrane upon startup and periodically thereafter. It can make a remarkable difference in performance and membrane longevity if your dissolved solids level is high. It can add greatly to the lifespan of membranes, and it can significantly improve the unit's TDS rejection performance.