Journaling Thoughts . . .

A man would do well  to carry a pencil in his pocket, and write down the thoughts of the moment. Those that come unsought for are commonly the most valuable, and should be secured, because they seldom return.
Francis Bacon

(Anyone who's name ends in bacon ought to be listened to!)

Another reason: those who don't keep journals, will be defined by those who do. Think about it.

The Middle-Aged Mormon Man gives a great synopsis here.