Paul is right. The insert is heavy porcelain and the flared base lent stability to the setup. If you have access to the 1975 publication The Cooks' Catalogue, you can view it as item # 6.67 on pg. 227. Some notes from the text: "Like almost every other traditionally designed example of this genre, the bottom pan is practically useless by itself because of the way the sides slant inwards: but this is a structural necessity because of the need to stabilize the weight of the extremely thick white porcelain upper pan." These date back to the time of solid fuel stoves when heat regulation was much less precise