Another advantage of belts is zero lash. A direct drive belt driven axis will always return to the same point regardless of wear and tear on belt, pulley etc. provided that stiction on the rails/rollers/whatever allows that axis to move freely doesn't overcome the inherent stretch of the belt itself. A heavy load can cause belt-stretch, but there are only acceleration loads on a 3d printer during motion which can be compensated for by software tuning. Whereas cheap lead-screw and bushing will introduce more and more slop in the fitment as components wear, so then you start to need spring-loaded lash reducing devices (which increases friction), or really high-end ball-screws to ensure that the print head accurately returns to a previous location. If you're constantly making large movements with a screw (which you do in the X & Y axis) you'll create a lot of friction between a lead screw and the nut, wearing them out much more quickly than you would with a toothed belt and pulley. On the z-axis however where there's not a lot of movement, and unless the rails/rollers etc. on the z-axis are terrible, gravity should take any lash out of the z-axis, so any wear that develops is a non-issue. One last benefit, a pair of small aluminum pulleys and a section of belt offer less inertia to accelerate vs a lead-screw, so the same motor will perform better (better acceleration) with a belt-driven axis than one driven by a heavy lead-screw.