Some photographers might use an ND (neutral density) filter in the day to obtain a wider aperture (narrower depth-of-field). Some use it to slow down the shutter speed when shooting a waterfall, for example.
I like using an ND Filter during dusk. The reduction in light allows me to use a very long exposure. When combined with a handheld shot in a moving vehicle, the lights will streak to become beautiful art. Try this only if you are the passenger.
Kuala Lumpur as seen from Jalan Loke Yew. Olympus OM-D, ISO 200, f3.5, 6 seconds with Hoya NDX 400 filter.