Escaped water. The primary drain takes the appliance's normal discharge; the secondary catches what leaks — a split hose, a weeping valve, an overflowing machine — and takes it somewhere harmless.

That matters most where the appliance sits above a finished space, because the alternative path runs through a ceiling.

It is insurance built into the wall, and like insurance it does nothing at all until the day it is needed.

Anywhere an appliance sits above something expensive or hard to dry out: upper-floor laundry rooms, apartments over other apartments, cupboards above finished ceilings.

It is also worth it where a leak would go unnoticed for a long time — a machine in a closed cupboard or a rarely used utility room.

At ground level on a tiled floor with a drain nearby, the case is much weaker, and a standard outlet box is usually adequate.

To a point where a discharge will be seen or safely disposed of, which depends entirely on local plumbing rules — sometimes a dedicated waste, sometimes a visible termination.

A visible termination has real merit: water appearing somewhere noticeable is how the occupant learns there is a fault at all.

Connecting it directly into a foul drain without the required air gap is a common mistake and can create a contamination path, so the routing needs checking against the applicable code.

No, and the two work together. A tray catches what drips from the underside of the machine; the box handles what escapes at the connections in the wall.

Leaks happen in both places, and neither component covers the other's case.

Where the consequences of a leak are severe, both are specified, sometimes with a leak detector as well so the fault raises an alarm rather than waiting to be noticed.

It needs one thing a standard box does not: somewhere for the secondary drain to go, and a fall to get there.

That has to be resolved at first fix, because retrofitting a drain connection into a finished wall means opening it up.

Beyond the drain routing, fitting is much like any recessed box — sized to the stud spacing, fixed to the framing, with the face plate applied after the wall finish.

Very little, but it should be checked rather than forgotten. The drain path can become blocked by dust and debris over years of doing nothing.

A box whose secondary drain is blocked provides no protection at all while appearing entirely normal, which is the worst of both worlds.

An occasional pour of water through it during other maintenance confirms the path is still clear, and takes a moment.

Only by opening the wall, because the drain connection and its fall have to be built in — that is the part that cannot be surface mounted.

Where the wall is coming off anyway, adding one is inexpensive and worth doing in any at-risk location.

Where it is not, a surface drip tray with a leak detector delivers some of the same protection without the building work, and is the usual compromise.