Rotating filter tray on a mattebox

[LuckyStudio 13]

Besides when using a polarizer on a mattebox, any other functions that being provided by the rotating tray or trays ? Is a rotating filter tray a must have for a mattebox ??

thanks guys

 

[j]

You use it for grads and ND grads. You can also use it for special filters like split field diopters or other effect type filters.