The Hama 58mm Vario Neutral-Density Filter ND2-400 has a number of benefits, not only does it protect your lens from dust and scratches but it also allows you control the amount of light passing through your lens by as much as 8 exposure stops.
By rotating the end of the lens you are able to control the amount of light (between +1 and +8 stop values) that enters the lens.
For instance, f/4 instead of f/8 for selective sharpness instead of great depth of field, or 1/15s instead of 1/60s for a smooth, blurred water effect instead of a ‘frozen' waterfall.