Let me add to what Ted said if I may. The increase in area as the mixture flows thru the spacer slows the speed of the air some what. That helps with the transition from verticle to horizontal as it turns toward the individual runners. It also helps keep the fuel droplets in suspension at the bottom of the plenum, rather than the miniture "rivers" that eventually wind up in the chambers, but in an uncontrolled way. I'm certain that this helps keep the air / fuel ratios more evenly distributed between cylinders.
The last thing would be an increase in the effective plenum volume which usually helps top end power, when it's needed. It's usually needed on a factory manifold on a modified motor.
John and Geoff Mummert know vastly more about this than I do. Like to see John's input on this subject.
Frank/Rebop
Bristol, In ( by Elkhart) 
