ND filters are used if too much light restricts creative possibilities. They reduce the light in accordance with their density. They, therefore, allow more widely opened apertures. For very bright situations the aperture would also have to be closed. This would mean losing the option of a short depth of field.
The Schneider Classic ND filters offer the highest possible color accuracy throughout all stops and in between different batches. This enables flexible filter stacking and reduces the color grading time.