From some quick research about undergravel systems for another thread, the benefit is a biological filter and less mechanical and chemical. The water runs through the sand where bacteria is to break down ammonia and nitrites and then goes to the bottom where a pump pushes the water back out.
I've also read about the gravel being packed down overtime causing flow restrictions. I personally would not use it because gravel leaves areas in between for water to flow, but sand would be more pack together thus limiting flow quite a lot. This is from pure observation of gravel and sand with no research involve though. I could be wrong