* Preventing Belt Slippage: Consistent tension prevents the belt from slipping on the pulleys. Slippage leads to inefficient power transfer, reduced accessory performance (like alternator output or power steering assist), and potentially belt wear and damage.
* Even Wear: Proper tension ensures even wear across the entire belt surface. Uneven tension causes premature wear in certain areas, leading to shorter belt lifespan and potential failure.
* Dampening Vibrations: The tensioner, often incorporating a spring or damper mechanism, absorbs vibrations from the engine and the rotating accessories. This helps reduce noise and prolong the life of the belt and other components.
The movement you see isn't necessarily large or dramatic, but it's a constant adjustment as engine speed and load change. The tensioner dynamically responds to these variations, keeping the belt at the ideal tension level throughout the engine's operating range. If the tensioner *didn't* move, the belt tension would fluctuate excessively, leading to the problems mentioned above.
