Why is it recommended to add gravel around pvc pipes that go underground to the water tank storage?

There are many reasons for laying pipe in bed of gravel. One of the major ones are:

To separate pipe from the dirt. If it is perforated pipe than it helps filter dirt and water gets filtered before it enters the pipe. Also gravel acts as cushion and absorbs earth movement as well as pressure from above, preventing pipe breaks...

Hope that explains... Back to you... Milo

Great explanation by Milo. In my area this is called, "bedding the pipe" Good luck. Tom