Except for pure 2-microphone stereo recordings, having a center channel feed/speakers removes from the L/R speakers most of the center information that would otherwise be mixed into them. That mixing is never perfect and the most obvious improvement on going from 2channel to 3channel is usually an increase in soundstage width and clarity due to the deletion of that material.
I can see soundstage clarity being enhanced by having the center channel information removed from the left and right front speakers, but how is soundstage width increased? Does it now extend laterally beyond the left and right speakers, whereas before it did not? And if so, what causes that?