It's a production thing. If you are building fine furniture you design
the edges to be exactly together and hand plane them to an exact smooth
finish, gently creating razor thin curls of wood as classiscal music
wafts slowly through the shop.

In the real world, you design them with a 1/4 overlap so as long as you
get withing 3/16 it's all good baby and you bang it on with your nail
gun as Howard Stern blasts out of the raspy stereo speakers in the
corner.