Basically the idea behind them is to get as many different herbs as possible in a confined area. The spiral and the subsequent hight differences mean that you create a number of different environmental conditions which normally would not be possible in a small space. The small area also means that they are ideal for a small garden and harvesting and watering is easy.
How it works is that you have a spiral bed in which to plant, this is held together with a structure of rock, brick or anything really that can absorb the heat of the sun through the day which in turn heats the soil. The top of the spiral gets more sun and has significanly more drainage than the lower parts, which informs your choice of planting. This means that you are able have thyme gowing almost next to mint or ramsons. The bottom of the spiral often incorporates a small pond allowing frogs and toads to breed and creating a wetter moist environment.