I think to allow wide players to get in positions etc.
My wide men do tend to find a lot of space when I've set Nigel de Jong to hold up the ball more often, what normally happens is he holds up the ball and this in turn allows my LB and RB, who are both attacking mentality, to get forward and make themselves available.
You tell your DM to hold the ball up to let the rest of your team to get into position. A bit like makelele used to do for chelsea.
Its useful if they have 'dictates play' or 'holds up ball' PPMs. If they are able to pass accurately, then they you can use them as a playmaking 'quaterback', who sits deep and fires passes all over the place.