Designers like to be clever and when I first saw this method of presenting one's portfolio, I thought oh, how clever! But I wasn't the only one, and now all those designers who know clever but can't or aren't inclined to come up with clever on their own seem to feel this is the most appropriate way to present one's portfolio pieces.
The hand-holding does bring human and dimensional elements to the image, pulling the viewer into the frame little bit, perhaps.
It also (sometimes) contextualizes the portfolio piece itself. This is particularly true for business cards, which get passed from hand to hand in real life.
It also creates, scale, of course. Posters look, well, big and postery.
It certainly isn't the most annoying design trend out there, but it's ubiquitousness is starting to wear thin. And I'm not the only one who thinks so:
I suppose, like all design trends, the technique has its place. I still like it.