I can only speculate, but here's my theory:
(a) it DOES change the flavor a bit - as anyone who's smoked a box-pressed vs cabinet cigar of the same brand & vitola can attest - I don't think it's drastic, but it IS noticable.
(b) I believe that when a stick is pressed, it squeezes out the excess moisture/oils within the leaves, compressing them and allowing the oils to sit on top of the leaves as apposed to within them. I believe this acts to improve the ignition of the leaf and promotes a more consistant burn throughout.
The effect can be dramatic, depending upon the quality & the oil-content of the leaf not to mention the length of time & how dramatic the press is...
Just my .02. :crosseyed