The UV/IR cut filter is an indispensable accessory to block unwanted infrared and ultraviolet light where the ZWO is still sensitive.
Note: Some ZWO UV/IR-cut filters may be labelled as only IR-cut, but they are exactly the same and will also block UV.
This filter increases the contrast, particularly in refractors, as the chromatic aberration continues in the infrared and ultraviolet spectrum. That is true also for apochromatic refractors where the chromatic aberration is generally only corrected for the visible light.