Every high end fitration system dreams big in its attempt at alchemy. The ideal would be to create a biological ecosystem that is self-sustaining and self-purifying. If we could create the "perfect" system all waste byproducts would be eventually converted to atmospheric gases (Oxygen, Nitrogen, Carbon Dioxide) and Water. Refining turds into these things without any undesirable byproducts is just about as easy as turning lead into gold, but still we try.
Any biofilter is designed to create an environment where a wide variety of Bacterial Colonies peacefully coexist in a symbiotic relationship, each performing its designated role in the food chain. Some eat solid wastes (fish poop) and convert them into ammonia (bacterial urine), which is then eaten by another type of bacteria and converted into nitrite (bacterial farts), which is then consumed by yet another bacterial cousin and converted into nitrate (more flatulence) which is eventually broken down into nitrogen gas, carbon dioxide, and oxygen by other bacterial action and plant life which take up Nitrates, Carbonates, and Water as part of their food chain (plants are carbohydrate consumers/converters too).
The ideal media, which BH attempts to be, offers an environment where the solids are thoroughly broken down (dissolved by injestion) over time which at least in theory keeps it from plugging off. The BH doesn't try to be a solids "filter" so much as it does a solids "digester".