It has been found the substrates used in most aquaria serve dual purpose of enhancing the beauty and affect the water chemistry, filtration and for the betterment of the inhabitants. The type of substrate you want to put in the bottom of the aquarium depends on the type of aquarium.
Some substrates like crushed coral and coral sand alter water chemistry as they contain calcium carbonate and raises carbonate hardness and buffer the pH. Crushed Limestone, Crushed Coral Skeleton, Coral Sand, Oolitic Aragonite and shell Grit serve the same purpose of raising carbonate hardness and pH.
Some substrates form a part of biological filtration system. The useful bacteria spread on all the aquarium surfaces, exposed to aerated water and even on the substrate. Other particles have high surface area and therefore substrates are involved in biological filtration. The Under gravel Filter and deep sand Bed make use of substrate for filtration.
Plants in the Aquarium require loose substrate for the roots to penetrate. The substrate you choose should always be chemically inert with smooth edges as sharp edges can harm the inhabitants.
