1988 LXi Timing Belt Tensioner Spring: Why is it there, or am I missing something?

I'm doing the timing belt. When I tighten the tensioner pulley bolt, the plate it's mounted to no longer is free to move, which has me wondering: Why is there a spring at all, or, what am I missing? Thoughts? Thanks.

From what I gathered when I changed my timing belt is its to give you some more manoeuvrability when installing and tensioning a new belt, didnt stop it from still being a pain in the ass to do though!

I think if you follow the Timing belt replacement book procedure you leave the tensioner bolt loose rotate the motor over three times and them tighten the bolt to hold the adjuster. So with the bolt loose the spring puts the tension on the belt. Just my idea.

Thanks old blue and sandwich. Following your advice, I loosely tightened the tensioner bolt (with the spring installed)rotated the engine, then tightened it after a couple revolutions. Was relived to find the howling I heard was the water pump I'd replaced a the same time.