Plugging Timed vacuum advance hose

Appreciate helping me understand or correcting me on what I am doing wrong. I have an edelbrock performer 600 on my la 318. I am using the timed vacuum port on the carb to connect to my distributor. Doing the timing says to pull the hose from the distributor and plug it. There is no vacuum in that line at idle. I don't think there is supposed to be vacuum in that line at idle? Why should I plug that line? I am sure I am missing something or doing something wrong.

Please educate me. I don't have a PCV hooked up yet, waiting on the grommet. If that matters.