Mechanical contact requires lubrication. Because conventional rolling element bearings rely on the movement of balls against a guide surface, lubrication is a standard part of ongoing equipment maintenance. But lubricant migrates to the guide surface, and also causes out-gassing, often effecting the process at hand or the environment. Maintenance is not only necessary, it can be frequent.
Porous media air bearings, on the other hand, require no lubrication. Because they ride on a thin film of air, there is never any oil migration or out-gassing.
Obviously, this significantly reduces maintenance requirements…a factor which should be considered in the total cost of the bearing contribution to any capital equipment expenditure, upfront and ongoing.