The most expensive part of a conveying system is often the conveyor belt.  It's of prime importance to make sure that the belt is fully functioning and free from damage.  That's what a self-aligning carrying idler does.  It ensures that the conveyor belts stay on track, thereby minimizing the chance of severe belt corruption.  Self-aligning carrying idlers are concave in shape and restrain the belt from running up and over the conveyor.  They also reduce the friction between the idler and belt.