Loose tools (transfer/supplementary tools) are not mounted in the electrode holder. Instead they are positioned separately under a tool attached to the electrode holder.
Depending on the size of the loose tool, you either use a press plate or a standard electrode when welding. Supplementary tools are used for welding special shapes or sizes that cannot be made with a fixed tool.
Sometimes it is also easier to place the tool in the right position on the material than to move the material into the right position under the tool.