All fittings require some form of “ferrule”. Some fittings have them built in and others require separate pieces. The “head” of the fittings are used to drive the ferrule and tubing into the receiving port and cause a deforming of the ferrules which forces the ferrule to grip on to the tubing and hold it in place. The “ferrule” part of the fitting (can be one piece unit or separate head and ferrule) is designed not only to grab onto the tubing but also to form a seal in the receiving port thus preventing leakage of liquid from the port. Some ferrules are designed to “seal” the port as well as grab the tubing in two places producing a more secure fit.