They were designed to allow you to fry food over the coals in a hearth, without having to stand there and hold the frying pan all that time. They both held the pan an optimal distance from the direct heat coming from a fire, and in doing so, also raised the pan up away from floor level to make it easier to work with the food items in the pan without having to bend down so much.
They can’t be used on modern stove tops, as the tall legs raise the pan too high above the heat source.
They seem to have first appeared around 1790, as recorded then in an advertisement in the The Pennsylvania Packet.
The first ones were sheet iron with legs welded on. By the mid-1800s, they were made of cast iron.
Literature & Lore
“I wake from dreams of polar ice, on which I’ve been a slider,
Like fishes dreaming of the sea and waking in the spider.”
— John Greenleaf Whittier (1807 to 1892). The City of a Day. 1843.
The name is an Americanism. Sometimes the word “spider” is still used in some places in America to refer to a regular frying pan.