tl;dr: Taking 60 seconds to jot some notes about your day can yield impressive professional benefits. Trailmix makes it particularly easy to build this habit.
Have you ever gotten an error and said "I know I've seen this before, but how did I solve it?". With a searchable journal, that annoying experience goes away.
The true test of a design isn't when you first write it, it's when you need to change it later.
If you make a note when your design choices work out particularly well or poorly, you'll soon build a catalog of techniques to embrace or avoid. After your third "Observer pattern on app_foo continues to confuse new developers coming on to the project", you'll learn to shy away in the future.
Emailing your boss a week before your review reminding him or her of your major accomplishments in the past N months is a very smart move.
As humans, we tend to have some good days and some bad ones. Because of this, it can be hard to detect trends in your mood. Consistent writing can help you detect that the bad days are consistently outnumbering the good ones. Maybe it's time to start looking.
Starting a development journal isn't hard. Maintaining one is.
The key to building a habit isn't initial excitement, it's consistency. The biggest enemy of consistency is forgetfulness.
If you'd like to try building this habit, you'll want to set a daily reminder for at least the first few weeks. When the alarm goes off, fire up your editor and record an entry.
Another option is an app we wrote called Trailmix.
Trailmix keeps you consistent by sending you an email each day asking what you were working on. You reply right in your email client and your entries are parsed into a searchable daily journal.
What's more, Trailmix includes an old entry chosen at random in your daily reminder. This keeps your previous entries fresh in your mind. You might never forget how you fixed an error again.