I'm not sure what you mean by "raised" rib...all vent ribs (or solid ones for that matter) are raised to some degree above the barrel. However, I do know what you are referring to when you say "step" rib...or it's close cousin the "ramp" rib.
Depending on your game, I suppose the purposes may be different. That is, I believe that trap shooters use rib/comb height configuration to make their guns shoot high in order to be able to keep a raising target in sight and still be able to hit it. Some serious trap guns have height adjustable ribs where the whole rib can be moved up or down.
We skeet shooters (that would be me), we tend to go for flat shooting guns. Amongst us, those that like step or ramp ribs do so because; when the comb height is appropriately set so that the shooter sees a "figure 8" configuration with the fore and mid beads, then the receiver is lower (further out of sight) in your view.
That is, no matter how high the rib is, with a Monte Carlo or adjustable comb you can get it to shoot flat (granted that you don't have anything going on with POI from barrel hangers, etc). If you do this with a ramp or step rib, the result is your face is higher above the receiver giving you a better look at the target.
I would be interested in hearing why others favor ramped or step ribs. I have been told that Kolar has recently quit making flat ribs in favor of ramp or step, perhaps an indicator of their burgeoning popularity.