[dictum]

Leaving common knowledge undocumented is, in itself, a misguided attempt at DRY.

Your common knowledge isn't necessarily everyone's common knowledge; what you perceive isn't necessarily what it is.

Every day there are many people just starting to code. There are cultural and regional differences. They might not have hit the exact spot in which this information appears.

Knowledge also disappears, fades, gets censored or destroyed, gets mixed up and remembered incorrectly. You might take something as a given and misremember it 20 years from now. You might read your own code 5 years from now and not know why something's there — something that was taken to be obvious.