The solenoid built into the vacuum advance permits a retarded timing setting at idle to attain correct idle speed with a relatively wide throttle opening for cleaner exhaust at idle. Power to the solenoid goes through a closed-throttle switch on the carburetor so as soon as the throttle is moved off idle, the timing jumps back up to normal. If you eliminate the solenoid, you might be able to attain a passable idle speed and mixture, but not a good one.
Read more
here (start at bottom of page and then move on to the next page).