The primary reason why most pros use steel shafts for their irons is because it twists lesser for off center hits e.g. hits closer to the toe, as compared to graphite shafts.
Then there's the weight factor, which affects or is affected by the tempo of the swing.
Technically, you could compress the ball against the turf better using a heavier shaft.
For iron shots, its all about striking down on the ball i.e. hitting the ball, then taking the divot.
But if you have a slow swing then your pro of fitter might suggest graphite shafts to get better club head speed.
Hope this answers your queries