If you buy these parts in bulk, they're not very expensive. And they work very well, both for gases and fluids and even for vacuum as long as you don't forget to put the stiffener sleeve in the pipe. Unfortunately you cannot re-use the compression ferrule or stiffener sleeve.

Use plastic ferrules on plastic tubing with the insert. The brass ferrule will cut and break the plastic line; the insert is needed to keep the plastic tubing from collapsing. With soft copper tubing: use a brass ferrule and no insert. Plastic ferrules are also referred to as Delrin compression sleeves.

Some people like to add a dab of RectorSeal around the ferrule edges. Buy the type for potable water and it will add one more level of leak protection. If there are minerals in the water, the very small holes that might crop up in the connection get filled fast (scaled over). You only have one shot at a good connection bc the ferrule is deformed in the process, so if you replace the brass ferrule, be sure to cut the line back a few inches, use a new insert, and a new plastic ferrule with a dab of sealant. Note that pure water applications (distilled, RO) require different strategies in the form of PTFE tapered threads or quick O-ring connections (John Guest type) bc the metals in the line will corrode.