I am only familiar with the Ford 7.3 powerstroke fuel system so I don't know if any of this applies to Chevy or Dodge, etc.
The Ford fuel system has some design flaws that can allow air into the lines from the tank to the pump. Due to the deadhead fuel rails in the heads, any air in the system can only be expelled by the injectors, which is not good for them. There are ways to fix this problem, but that would be a whole different thread.
The FASS system works by pumping the fuel from the tank through water and air separating filters and then sending the fuel at a low pressure to the main fuel pump. This takes some of the load off the main pump and provides clean, air free fuel to the engine.