If the holding tank for your waste oil varies, the pressure of the oil to the nozzle will change. This change in pressure will be small, but can greatly affect a siphon system. A full holding tank will send more pressure and as the tank empties, the pressure will reduce. By having the oil enter a CL tank, the level will always be the same, therefore always presenting the same pressure to the siphon nozzle. Actually you usually want the oil level in the CL tank lower than the nozzle. If it were above, I would imagine the pressure would force oil out the nozzle while the burner was shut down, causing a mess.
I believe the commercial units use a metering pump to deliver a constant and steady flow of oil to the burner. In a siphon system you either need a CL tank or a metering pump. Metering pumps can be expensive, so many opt for the CL tank.
