The Dummy job entry does nothing. It is just an entry point on the canvas; however, suppose you have a transformation that processes one row at a time. You have set the transformation so that it gets the initial five records and processes the additional records five at a time. The Job script must determine whether or not processing is complete. It may need to loop back over a few times. The job workflow drawing could be tough to read in this type of scenario. The Dummy job entry makes the job workflow drawing clearer for looping. Dummy performs no evaluation.
Warning: Although it is possible to create a recursive, never ending loop, you should be aware that such a job will fail with an out of memory or stack error after too many iterations. So it should exit after a specified number of loops. It is much better to design the loop differently, for example by scheduling within the DI-Server in a repeated manner or by the Start job entry and setting the interval.