I cant attest to his specific reasons for this practice but I can mention some reasons I see that it would be useful.
When I first started shooting I often had my hand on the slide for at least part of the forward travel. This doesnt allow the spring to do what it is supposed to in returning the slide to its proper place and on more than a few occasions I induced a malfunction.
More recently, my wife was handling a pistol and while chambering a round part of her palm was pinched between the slide and barrel. As I recall it took more than a week to heal all the way up.
By holding your hand in place on the slide and "running" the pistol forward under it, neither of the above problems would occur.
"Be kind, be courteous, and have a plan to kill everybody you meet."