The gaiters locate between the brake caliper (the moving part by the wheel which grabs the wheel to make you slow / stop) and the brake cable - helping to prevent dirt and water thrown up from the road from getting into the inside of the cable outer.
why are they useful? The inside of the cable outer, and the brake cable itself are made from steel. Water around the cable can cause rust. Either rust or dirt / grit surrounding the cable will make it less efficient - meaning your brakes won't work as well as they should.
in addition to using these gaiters we recommend spraying a good water disbursing lubricant such as GT85 or MO94 inside the brake cable.