Manually racking the slide (assuming proper technique) offers slightly more forward energy (by adding a tiny bit to forward slide travel and spring compression) and therefore provides a slight reliability benefit over dropping the slide using the slide release/stop.
In most cases, it's not enough of a difference to be significant, but it can make a difference between a malfunction and proper operation in some cases.
I entered one of my Ruger P95 pistols in a 1000 round match some years ago--this is the match DNS is referring to above. On round 970, due to fouling buildup, the pistol failed to chamber a round from a full magazine when the slide release was used. I kept shooting the pistol and it operated perfectly for another few hundred rounds (until I ran out of ammunition) as long as I chambered the first round manually by racking the slide. It would consistently fail to properly feed the first round if the slide release/stop was used instead.
The primary reason I rack the slide instead of using the release/stop is because one of the guns I occasionally carry does not have a slide stop/release and I like having one procedure that works for all my guns.