Daytrip

I bought the Park Tools torque wrench (the larger one) as part of my winter project of replacing the groupset on my Giant OCR-1 with the Ultegra group. I'm wondering why the wrench handle ratchets back and forth. It's obviously designed to do that (there's one pin that holds the handle to the main shaft of the wrench), but I can't figure out why.

I should add that I've never owned a torque wrench before.

I know some people never use torque wrenches and I respect that, but I figured if I'm going to do this, I ought to at least attempt to do it right. As such, I'll probably now try to torque down everything in sight. What would be the most critical parts to torque down on a project like this? Is it a good idea to do the pedals, for example?