I learned about the importance of sand (and other media) this year when setting up a saltwater tank for my s.o.; I really had no idea beforehand.

Sand provides a huge amount of surface area for colonies of bacteria responsible for converting ammonia into nitrites and nitrites into nitrates, which can then be taken up by plants or algae and converted into other chemicals that are less harmful to aquatic species.

Dredging sand from rivers and beaches is likely going to be seen as a massive ecological disaster in the next couple of decades.