The idea with the liver is to provide the right bacteria to break down the solids and to keep the right kind of bacteria in the bed to keep it open and not get plugged up with bio-film.
There are 2 kinds of bacteria at work in a good system, the anaerobic that do most of the work in the tank an seeing as they work without oxygen that is a good place for them, but then you want some of those as well as aerobic bacteria, they need oxygen, in the bed. The anaerobic do the heavy work but leave a bio-film that will plug up the bed, so that's where the aerobic boys come in and you need lots of them, and these are provided by the liver. You could also inject air into the bed or tank, that works as well but may smell a bit at times,, mind you, a good working tank won't smell much, if at all,, so if you smell your tank, time for liver!