In mold making or reverse engineering of products, there are times when you need to fill in a negative space with a solid. Normally, if all you want to do is fill the space, extrude/rotate/sweep/loft, etc will work. However, there are times when I need to use the negative space as a tool body later on. If there is a fill void command, i can use that as the copy of the internal space, then apply a new operation, like threads or something that will penetrate the internal void. Then by using the void solid as a toolbody, can return the internal geometry to what it was. Often times, the internal geometry is too complex to simply extrude or rotate it away. Think an injection molded part that has been created first with external geometry then shelled to represent the consistent wall thickness. The new internal void geometry may be cumbersome to recreate with the commands on hand.
