My purpose for the relay is so that I can have Mach turn the router on when it starts a job. I had planned to put an M3 command at the beginning of the G-Code file to do that. However, rather than edit this command into the file, I thought why not have a Brain turn the router on for me anytime I perform a cycle start. The router has an on/off switch, so I will turn that off anytime I am changing bits.
If a person plans to start cutting anytime they click cycle start, then I think the Brain approach makes sense. However, in practical use, I did not know if people hit cycle start for other reasons. The only one I could think of is if you plan to cut air without a bit and without the router turned on. I imagine that is rarely done.