They are cheap, and simple but they stop your house from falling down.
Cavity walls were invented around 100 years ago. They provide an enormous advantage over solid walls in terms of thermal efficiency. Basically a house with cavity walls is warmer than a house with solid walls.
However two separate courses of bricks are weaker than bricks that are joined together. So the question was asked how do we fix together the cavity wall inside and outside to each other without breaching the thermal efficiency.
The answer was to join the two sides inside and outside together with a cavity wall Tie.
Or more properly, join them together with lots of cavity wall ties. Cavity ties are mostly made from stainless steel and are designed to be in embedded into the mortar as the brick courses are being laid.
The centre of the wall tie includes a drip formation. This is to stop any water from travel along the tie and bringing dampness into the building. Any water that does start to travel is caught by the simple wiggle in the steel rod and drops harmlessly before reaching the inner wall.
Older buildings before the late 1970s had galvanised steel (or even plastic) ties but stainless steel is far less prone to rust or degradation in the long term.
The Ties should always be placed either perfectly horizontally or with a slight downward slope from interior to exterior wall.
Wall ties are cheap to buy and quick to place so there is no incentive not to use them. The dangers of not placing Ties when building a cavity wall are basically the outer (non load bearing) wall simply falling away.
Where should you place the ties.
Very clear guidance has emerged over the decades and any builder should be referencing to local regs when putting up a wall. Particular attention needs to be paid to location and quantity of wall ties around cavities (doors and windows).