Three things, and each is a reason the tube is chosen.

It gives shape retention. A plain plastic tube springs back when bent and has to be clipped frequently to hold a route. The aluminium layer is ductile metal, so a bend formed by hand or with a bending spring stays formed. That makes the tube behave like soft copper on site - it can be routed neatly and left.

It reduces thermal expansion dramatically. Plastics expand a great deal when heated, which on a long hot run means noise, movement and the need for expansion loops. The aluminium restrains that, so the composite tube expands far closer to metal than to plastic.

It is an absolute oxygen barrier. Oxygen cannot diffuse through metal, so no EVOH layer is needed and no oxygen reaches ferrous components in a sealed heating circuit.

It is easier to route and harder to prepare, and the trade is usually worth it.

PEX and plain PE-RT are more flexible and cheaper, and they come in long coils that suit continuous underfloor circuits with no joints. But they spring back, so they need frequent clipping to hold a line, and they expand substantially when hot.

Multilayer composite holds its bends, looks tidy in exposed work, and moves much less with temperature. That makes it the better choice for visible pipework, radiator connections and services in ducts and risers.

The cost is preparation. Every cut end must be reamed and calibrated before a fitting goes on, which PEX largely tolerates without. Skip that step and the aluminium leaves a burr that cuts the O-ring in a press fitting, and the joint leaks - often not immediately.

Because cutting distorts the end and leaves the aluminium edge exposed, and the fitting seals against that end.

A cutter squeezes the tube slightly out of round as it cuts, and it leaves a burr on the inside where the aluminium and inner polymer are severed. A press or compression fitting relies on the tube being round, the right diameter, and smooth inside where it slides over the fitting's insert.

A calibrating tool does both jobs at once: it re-rounds the end to the correct internal diameter and chamfers or reams the bore so the insert enters without catching. Without it, the insert drags across the burr and can cut the sealing O-ring as it is pushed home.

That failure is insidious - the joint often holds initially and weeps weeks later, buried in a wall. It is the single most common cause of leaks on composite tube systems, and it takes seconds to prevent.

Yes - it is one of its main applications, with the temperature capability coming from the polymer layers.

PE-RT and crosslinked polyethylene inner layers give the raised temperature performance needed for hot water and heating circuits, and the reduced thermal expansion from the aluminium makes hot runs much better behaved than plain plastic.

As with any plastic pipe for hot service, the rating is expressed as an application class combining a continuous operating temperature, periods at higher temperature and a malfunction temperature, all with a design pressure and design life. Select against the class for the system - underfloor heating, radiator circuits and hot and cold plumbing each have their own - rather than against a single temperature figure.

The aluminium barrier means no separate oxygen barrier layer is required, which is a genuine advantage in sealed heating systems with steel or cast iron components.

Bent once or twice carefully, yes. Repeatedly in the same place, no.

The aluminium layer is what holds the bend, and it does so by yielding - the metal deforms plastically. Working the same spot back and forth work-hardens it, and eventually the aluminium cracks. The polymer layers may hold pressure for a while afterwards, but the barrier is broken and the tube's behaviour has changed.

So form a bend once, using a bending spring or former for tighter radii to avoid kinking, and respect the minimum bend radius. Do not straighten and re-bend a section to correct a route - cut it out and start again.

Sharp kinks are permanent damage regardless of how they look afterwards. A kinked section should be removed rather than straightened, because the aluminium has already been damaged at the fold even if the tube appears round again.