It might help to understand if you think of the sandbed as a ecosystem in and of itself that provides the primary filtration for your aquaria. It, just as much as your fish, must be fed and appropriately cared for---after all, there are millions of organisms in the sandbed---as compared to the fish you have placed in your tank. This is a symbiotic relationship between DSB and the desired organisms that you place in your tank.
The DSB’s food sources are the waste byproducts of your fish & other animals. The animals that you desire to place in your tank [like fish] produce food for the animals in your tank that you will never see [infauna bacteria & microalgae], and the animals that many hobbyists [mistakenly] want to rid themselves of [like worms].
The breakdown of these byproducts in to nitrogen gas occurs in the lower regions of the DSB were anaerobic zones [low oxygen] exists. Bacteria growing on the surface of each grain of sand [within the DSB including these low oxygen zones] is the “export” mechanism which facilitates this process. Thus, you see the importance of cultivating and, if you will, protecting this bacteria. Bacteria in the DSB must not have competition for either space or food, else you diminish the DSB as a filter mechanism. Competition for food & space occurs with sand disturbing organisms such as gobies—that eat the sandbed infauna as well as continually disturbs the bed with their tunneling [as well as swallowing and spitting out the sand resulting in wiping out the bacteria population on sand grains] resulting in DSB bacteria mortality. You can see why I said, “NO WRASSE” in the DSB. The sandbed must not be infiltrated except by infauna organisms that will work in harmony with the sandbed bacteria, slowly and gently overturning the bed so that the population of sandbed bacteria and microalgae grows, thrives and become diverse. If the sandbed ecosystem is constantly being disrupted or preyed upon by other organisms, then the DSB is, at best, not working optimally, and, at worse, dysfunctional.