Bearings reduce the friction that hinders the movement of the gears, which means they reduce power consumption and improve the rate of fire. The bearings are especially suitable for stock power High Speed builds. These should not be used in any super straining builds, like DMR or DSG, because their small size makes them less durable to the strain caused by e.g. M180 spring.
9 mm bearings go well with guns that have around 1.7 J muzzle energy, as long as the rate of fire is not too high like 40 rps.
RetroArms – CNC Engineering at Best
RetroArms is a Czech airsoft company which manufactures high-quality upgrade and spare parts. The company has become known from its CNC machined parts, especially its super durable gearboxes which are made from 7075 aluminum. RetroArms gives its gearboxes a lifelong guarantee against breaking which tells us a lot about the quality of the parts!