The drive belt runs your power steering, alternator, water pump and your AC compressor. When the PCM (Power Control Module) activates the AC compressor clutch, this event puts a load on the drive belt and the tensioner. This event could aid in determing if the tensioner needs to be replaced.

A tensioner that bounce or oscilate back and forth does indicate a concern. If could be a concern with the belt itself or a siezing pulley. This is the reason a kit is recommend when doing a repair, which also reduce the guessing game.

If the belt walked, in my expereince of doing this type of work; THERE IS A REASON WHY IT HAPPENED. Find that reason before performing any repairs.