What is the purpose for putting gravel or rocks in a fish tank?

like jill says, the gravel helps to trap some of the waste and debris, but the big thing is that the gravel also holds bacteria that can be beneficial because they break down the waste and the harmful chemicals in the water. you can have a tank with no substrate, but you'll have to watch the water levels better and have a great filtration system. bare-bottom tanks are actually pretty popular among some aquarium hobbyists.

gravel is pretty with a purpose...it also helps trap waste so everything isn't floating around. if you ever have a fish bowl without it, you will see what I mean, its really messy without gravel