Because almost nothing inside it is rated for gas. The valve, the seals and the connection thread are specified for water service, and gas requires components approved for fuel gas with the appropriate marking.

Thread forms differ too, so a gas appliance connector will not correctly engage a fitting intended for a water supply, and forcing it produces a joint that may hold briefly and leak later.

This is one of the clearest cases in plumbing where two products that look similar on a shelf must not be substituted.

In most jurisdictions gas work is restricted to registered or licensed operatives, and connecting an appliance to a gas supply falls within that restriction.

The requirement is not a formality: the installation must be pressure tested and, depending on the market, certificated before the appliance is used.

Buying the box is unrestricted; installing and commissioning it generally is not, and the applicable national scheme governs who may do the work.

Behind a cooker, a gas dryer or a gas fire, where a projecting elbow and flexible hose would stop the appliance sitting back against the wall.

Recessing the outlet recovers that depth, which in a fitted kitchen is often the difference between an appliance that lines up with the units and one that does not.

It also protects the shut-off from being knocked, while keeping it reachable without moving the appliance — which is the point of having it there.

Yes, and it is usually a requirement rather than a convenience. The isolation valve must be reachable without moving the appliance or dismantling anything.

That is precisely why the box is recessed rather than buried: the face plate keeps the valve visible and operable while the pipework sits inside the wall.

Boxing the assembly in behind a fixed panel afterwards defeats the design and would normally fail an inspection.

Typically a steel or rigid polymer housing built to suit standard stud centres, with a removable face plate. Where fire compartmentation matters the housing has to be rated accordingly.

Unlike a water box there is no drain requirement, since there is nothing to catch — which is one of the visible differences between the two products.

Some designs include a gasket at the face plate to limit air movement through the wall penetration, which matters in a room with mechanical ventilation.

Yes, though it is a chase-and-make-good job rather than the straightforward stud fit the product is usually designed around.

The depth of the chase has to accommodate the full housing, and cutting that depth in a structural wall may not be acceptable without checking.

In solid-wall situations a surface-mounted outlet with a shallow cover is sometimes the better answer, accepting a small projection rather than weakening the wall.

By pressure testing the gas installation as a whole, to the procedure and duration the national scheme sets, before any appliance is connected and used.

A drop in pressure over the test period indicates a leak somewhere in the tested section, and the box's connections are one of the joints under test.

That test is the reason gas work is certificated, and it is the step that distinguishes a completed gas installation from a merely assembled one.