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Is there any benefit to having sand instead of gravel in a saltwater aquarim?

Most of our customers use aragonite sand,
because lots of fish and critters prefer to dig in sand.
Bigger crushed coral is okay too.
I recommend a relatively thin layer, about a half pound per gallon,
or about a half to one inch thick
(easier to keep clean.)
Certain fish and critters (Sea Pens for example) like deep sand.

To some extent, a calcium based substrate will also help to buffer the water. This helps keep the pH stable, however, most systems still require the addition of a carbonate buffer (dKH buffer, reef buffer, etc) to keep the pH constant between water changes.

In general, if you are looking to go with a reef tank in the future, you want to use a sugar-sized aragonite. Crushed coral and gravel will trap more detritus and nitrates (and thus algae) will become a problem. Also, corals such as plates and open brains (Fungia, Trachyphyllia, Wellsophyllia) are best placed on sand becase the flesh is very delicate and can be torn around the edges if it has to open and close against rockwork or course crushed coral.

If you are adding sand to an already established tank, live sand or "aragalive sand" in the bag are expensive but very clean. You can pour the sand through a wide-dimeter pvc pipe to direct it where you want. Live rock should sit directly on the bottom glass so it doesn't shift and collapse when animals dig.