Also includes some nice youtube videos inline for context.
They're most often used as measurement references. You inspect parts on a surface plate (table or block that's very flat) using a dial test indicator on a rigid base that can be slid around. The indicator has a very short range of measurement (say, 0.01" for an indicator with 0.0001" graduations), so to inspect a part surface that's supposed to be 1", you'd wring together a 1" stack of gauge blocks on the surface plate, adjust your indicator up or down on the base so that its probe is in the middle of its travel on top of the stack, and zero the indicator face. Now you can measure 0.9950-1.0050"... just slide the indicator stand to your part and sweep the indicator probe along its surface to measure.
Guage blocks are for measuring things. The "wringing" thing has been used in optics for precision alignment for centuries, though nobody calls it that.