Much more than just the "stuff on the bottom of the tank", the substrate is an integral part of any saltwater aquarium system. Besides contributing to the visual appeal of a tank, substrate mediums also serve other purposes:

• It is a vital part of the biological filter base of an aquarium, and if you choose to use a non-living media over live sand, it will become "live" as the tank goes through the cycling process and the aquarium matures.
• Fine grained mediums are used for NNR (Natural Nitrate Reduction) in conjunction with denitrification filtration set-ups, such as plenum, fluidized and deep sand bed filters, while courser ones may be used in wet/dry trickle filters, or when a UGF (Undergravel Filter) is present.
• It provides refuge for many types of bottom-dwelling marine fish and invertebrates that either build burrows or bury themselves in the substrata, like various species of Wrasses, Gobies/Blennies, Mandarinfishes, Shrimps, Starfishes and such. A somewhat fine to small grained medium is good for these types of animals.
• Certain types of sand shifting invertebrates ingest substrate and extract organic matter from it, like sea cucumbers. Animals such as these rely on the substrate as a source of nutrition, and therefore a rather fine grained medium should be provided.
• The substrate houses other smaller critters, such as amphipods, copepods and marine worms, which are a source of food for some other marine animals.
• Natural ocean mediums are calcium-based, which is beneficial to saltwater systems in many ways.
• A calcium-based media of the right consistency makes a good material for use in  a calcium reactor.
• It is a material used for making DIY rocks.

When deciding on what "consistency" of substrate to buy, as you can see one has to not only consider what it is going to look like, but factor in what it is going to be used for, if it will be appropriate for the kind of system you are setting up, and conducive for the particular types of marine life that will be kept in it.

One last thing to consider is "where" the medium comes from. Sand (like the type used in a children's play sand-box), gravel (such as the kind found in fish stores that comes in pretty colors that is primarily sold for freshwater tanks), as well as rocks that are quarried on land often contain unwanted minerals, metals, silicates, and possibly chemicals or toxins that may leach out into the aquarium water and become problematic. Therefore, land-based materials are not recommended.

The best kind of substrate to choose is one that is naturally high in calcium content that originates from the ocean. Aragonite, coral/reef, and shell-based mediums are the top choices, which can be bought in various colors such as pink, black, white and neutral tones, and come in a wide range of consistencies from very fine to small grain, and medium to course textures.

Tip: For aquariums where no "sand" sifting or dwelling animals are present, a somewhat large or course textured shell media can be very eye appealing, such as in a specific fish-only predatory tank community that houses Triggers, Lionfishes, Puffers, Hawkfishes, Eels and such.