Next time you drive your car when you get
home turn the engine off and grab the fan belt.
It will be hot , very sticky and often leave a black mark on your hand
Feel your tyres and they will be similar, but not quite as sticky.
NExt time you mow, when you have finished, turn off the mower & grab the belts.
The covered belt will be hot but barely sticky at all.
The aim of the cover is to allow the belt to slip when required to fo so .
In particular when you hit the brake on a tractor style mower or dissengage the blades on a manual PTO mower of any kind.
These are
designed to slip without damage to the belt when the tension is let off.
Some ZTR's use raw edge belts on the drive because it never goes slack as the trannies on them are constructed differently to tractors.
The other reason for covered belts is to prevent them from picking up debris while mowing