Because Lego blocks are machined to extremely high tolerances, you can use them for quick, cheap but very accurate jigs, perfect for gluing, squaring, molding, etc. Here is an example of how Jef Raskin, who taught me the trick, used them. (Sadly Jef died of cancer last week). He built up the exact jig positions by stacking bricks of various thicknesses. In the case shown below he built up a jig to square up wings on his radio control model airplanes. All you need is the large Lego baseplate glued to a heavy duty flat foundation.
The largest Lego Baseplate is Gray, 15 inches (38 cm) square.
Jef Raskin was the leader of the original Macintosh computer team, also a fabulous connoisseur of exotic musical instruments, a pioneer of radio control planes, and he had an amazing workshop. An archive of his site is available and very useful.