Keeping a logbook is a part of experimental physics.  In grad school, I learned "If you don't write it down, it didn't happen." 

  And a diary is easy and fun - just a sentence or two every day (even on a wall calendar) reminds you of experiences, travels, travails, and the daily whatnot of life.

  Useful?  Sure!  My lab notebook and home diary were prime ingredients in writing m'book, The Cuckoo's Egg.