Here are some thoughts from Jim Rohn on keeping a journal:
Be a collector of good ideas, but don’t trust your memory. The best collecting place for all of the ideas and information that comes your way is your journal.
The reason I spend so much money for my journals is to press me to find something valuable to put in them.
There are three things to leave behind: your photographs, your library and your personal journals. These things are certainly going to be more valuable to future generations than your furniture!
Don’t use your mind for a filing cabinet. Use your mind to work out problems and find answers; file away good ideas in your journal.