The fireplace has changed quite a bit. Look at a house built in the 1700s/1800s. A fireplace was a giant open hearth, maybe 4ft tall. It was this way because you used it to cook, and it was used as the primary for heating. As central heating became more common in regular homes, the fireplace became smaller until it was nothing but a showpiece that will suck more heat out than it puts in to to the room. In order to get the same benefit our great-great-great grandparents did, we need an insert that doesn't allow the heat being generated to suck itself up the chimney. But it's still a brick hole in the wall that you burn shit in, so it's primary method of operation remains the same.