After we set up our tanks and put them through their nitrogen cycle, they begin to go through another cycle until they reach an equilibrium. This takes a loooong time. Months and months to the point where the natural habitat we've tried to emulate starts supporting itself. I think dirt helps expedite that process by providing a near infinite source of nutrients and minerals that support the tank throughout its life. Making our job as caretakers quite a bit easier since we don't necessarily need to fertilize or mess with our water chemistry. If we just sit back and let our simulated nature grow on its own, the results are often remarkable. Now this is more of a make it and let it sit sort of aquascaping, whereas many aquascapers will constantly fiddle with their tank. Both have their attractions, but for someone like me, who always has the next project on his mind, this method is vastly more enjoyable than constantly pruning one super high-tech tank.