I think that this is the image that you are referring to.
These are for low rpm situations. The twists of the belt cause it to heat up, and centrifugal force at high speed causes the belt to jump off the pulleys.
There is also friction of the belt rubbing against the pulley sides which adds more heat and wear. See the second image of figure 18 at point "D". The portion of the belt at the rear of the lower pulley leaves at an angle, with lots of rubbing and friction.
I used to own a 1957 International tractor that had the old style pulley power take off at the back of the machine, mounted on an extension of the transmission shaft, so it was was on a vertical plane perpendicular to the center line of the tractor. I ran a Woods belly mount mower deck on that tractor that had three rotating blades, all operated by horizontal pulleys. The center blade had a three-stack pulley. Two "idler" pulleys were mounted on the tractor, below the power take-off at odd angles. The single V belt was about 18' long, and traveled from the power pulley, over the left idler, to the bottom of the center stack, to the right blade, across the middle center stack, around the left blade, the top of the center stack, the right idler, then back to the power pulley. The belt lasted maybe four seasons for me, or about 100 hours.
Older cars used multiple V belts, usually three, in three different vertical planes. The crankcase pulley was a multi-stack. The accessories had to be lined up perfectly to avoid excessive belt friction, and brackets were adjustable for that as well as to tension the belts. Larger V belts developed more internal friction since they bend then unbend about their depth with every rotation around a pulley. As they wore and stretched the accessories had to be readjusted to keep tension on the belts, and the belts had to be elastic to allow this. Belts lasted maybe 50k miles, about 1500 hours.
Modern serpentine belts reduce friction due to reduced depth. The multiple V shape means that they can be as wide as needed for the intended power transfer. Idler pulleys configure the belt in a path to serve multiple accessories, so normally a single pulley can serve them all. A tensioner pulley allows the belt to be made from relatively inelastic materials that last 150k miles or more, about 6000 hours.