Seems like there may be some terminology confusion.
The sleeve that goes over the tubing and into the fitting and nut with the beveled edges is called a ferrule - it is required for all compression fittings no matter what the tubing material.
The insert that goes into the tube is required for nylon tube to keep the tube from deforming under compression. It is required for nylon tube but not for steel or copper tube.
Also, if you are reusing the fitting and nut - make sure the ferrule shape and dimensions match the ones you already have or you will have leaks. Not all ferrules are made the same from different fitting manufacturers.
YMMV, Greg