As Steve says, that's probably for a solid flywheel conversion. The DMF acts as a torsional vibration damper and so makes the engine smoother (Theoretically). However I don't think you'd notice a whole lot of difference in everyday running. My old Xantia HDi didn't have one to the best of my knowledge, neither did my HDi Picasso and they were acceptably smooth, to me at least!
2008 2.0 TDCi Zetec