I believe that the reason is... a racking spot is fairly thick, relatively speaking... and when you have a racking spot on the table, it elevates the 1-ball above the playing surface... and when the CB comes in to crash the 1-ball, it is doing so from beneath the 1-ball... the two ball centers do not match up vertically... meaning energy is being wasted that would otherwise go into breaking the balls if there were no spot on the table... no spot, the centers of the balls align vertically.
I get the best of both worlds by using a paper hole reinforcer as my spot. That way, the 1-ball rests on the table (no elevation) yet the hole reinforcer acts as the locator for the 1-ball to spot. And the hole reinforcer is way thinner than a typical spot for shots other than the break.
One day I'll post a picture of my easy rack indexing system. I dreamed it up when I first bought my Diamond table and Diamond rack. I get perfect racks every time.