Absolutely agree with Culzean, what is generally termed "poly-V" belts were a game changer for front-end auxiliary drives on engines.
You can get them with poly-V on both sides, though not that common, and as Culzean says, the smooth backside is generally used for the lower demand stuff, like cooling pumps. The limiting one is usually the alternator, small diameter means lots of flex angle which stresses the polymer a lot. The cords do all the heavy drive work. Big alternators with small pulleys usually have over-run bearings to allow them to spin on after the engine slows abruptly to avoid slippage, which destroys belts quickly. A high drive ratio alternator has a big effective inertia, running at maybe 4x engine speed.
A single serpentine run with an auto tensioner can save considerable cost in assembly and manufacture, plus it improves reliability and longevity, minimising costs.
There are "stretchy" poly-V belts now, for alternators etc, which don't need mechanical tensioning, but they are limited on power transmission (belt tension has to be greater than drive tension force to avoid it coming "loose"). Essentially the proverbial rubber band.