1)it allows you to see your fish, with out light you cant see anything.
2) the different hue of light you have will reflect off your fish making them show up more of that color, sometimes it looks better sometimes it doesnt
3) some types of fish have night coloration, where if theres not enough light they look faded, mainly saltwater fish
4) some times the intensity and wavelength of the light will change the fishes actual color, example goldfish put in sunlight may develop a different color then they had in the aquarium, probably the equivalent of a tan but much more drastic