These were originally bought for sugar-craft to help support the structures, but now vermicelli or spaghetti pieces tend to be used instead.
Our Nails are now mostly used for equalising the heat distribution in the centre of cakes when baking, as a 'cake spike'.
If the cake is a bit larger, perhaps a four or five egg sponge, it is not unusual for the base and centre to cook more slowly than the top, or the sides, or both. Place the nail head down on the base of the cake tin in the centre, press down the greaseproof/brown paper over the spike of the nail (carefully) to line the tin as usual, and pour in the mixture. After baking, you can easily remove the nail when you place the inverted cake on to the raised grid to cool as usual. An even bigger/thicker cake might require two or three or four evenly spaced nails.
Also useful for controlling the temperature around the edges and top are extra layers of greaseproof/brown paper outside the tin, these help to slow down the heat transfer from the oven, especially useful with a fan oven.
This trick is well worth a try!