"If you can't explain what you're doing to a 6-year old, you don't know it yourself" (ascribed to several famous physicists).

This is pretty true, and thus the exercise of explaining things in plain prose is very useful to make you polish your understanding, and find any problems in it. When hand-waving is not acceptable, problems in understanding become very visible.

Even if nobody reads this, it's very useful if node honestly. Having other people to read it just keeps you from slacking off.