Adjustment of center distance or addition of an idler pulley is crucial to compensate for wear and stretch.
From the en.wikipedia.org
The entire belt may be tensioned by a single idler pulley.
From the en.wikipedia.org
If the engine belt idler pulley bolt breaks, the vehicle may unexpectedly lose power-assisted steering, making it suddenly harder to drive and increasing the risk of a crash.
From the latimes.com
The powered pulley is called the drive pulley while the unpowered pulley is called the idler.