This is a kit for a simple test instrument called a logic probe. It allows you to see the state of any pin in the computer while it is running, without interfering with the operation of the computer. It is not needed to run the computer, but it is an educational feature of the kit. A logic probe is an essential tool for use in building and debugging prototype circuits, such as the wire-wrap version of this computer, since with many hundreds of connections you are bound to make some mistakes. But, for the computer kit here, that uses a printed circuit board, it is rarely needed. You might forget to solder some connections, but you should be able to find those mistakes by careful inspection.
Besides the educational feature of using the probe to look at the states of the pins of the computer, it also serves as a good introductory kit for building the other more difficult kits of this project.