They have four main uses: 1. Enables slower shutter speeds to be used. To display movement instead of freezing motion. Example, creative waterfall shots. 2. Decreases depth of field. Effecively blurring the background. 3. Reduces the amount of light reaching the sensor, but with no affect on colour balance. Reduces light intensity, (sunshine, snow etc). 4. Allow video cameras to operate in bright light. Brand new in package with clear acrylic case.