PEX fittings are used to connect the flexible polyethylene pipes used in PEX water plumbing
The flexible pipes are not made brittle by low temperatures. Polyethylene tubes are cheaper than metal pipes and are easier to install because they do not use as many fittings as metal or rigid plastic systems. Since soldering is not used with PEX systems there are no nasty fumes or the risks associated with using a torch.
PEX tubing is wrapped in spools. Because spools weigh less than cut metal or plastic pipes, they are easier to store and cost less to ship. Tubing can be unrolled from the spools with no need for coupling fittings. Installation is much quicker and cheaper when it does not involve the running of pipe.