Steam cleaning also produces less runoff and less splatter. Even with a hot water pressure washer, you will splatter any oily residues that you do remove, and there will be a lot more water runoff.
Professional pressure washers, the reduced runoff with steam cleaning means less reclamation to deal with. In most city many big jobs will require that us to reclaim the runoff water to conform to EPA regulations.
The main advantage of steam cleaning is the heat. The heat provides faster and more effective sanitation, getting the job done quicker with less water. Chemical detergents work better when paired with higher temperatures, emulsifying the dirt and oil much better. This results in a deeper clean with no oily residue at all, which is perfect for surfaces that you plan to paint or refinish. And with no oily residue, surfaces will stay cleaner longer.