I keep one of sorts, it is part of my monthly homework. The journal is very basic and includes what trades I made during the day and my reasons for getting into the trade. My journal has changed a bit over time since my reasons for entering a trade now are pretty consistent. I think there are three reasons for keeping a journal.
One reason is to have a manual backup in case there's a system or software failure. I had to take advantage of this a couple of times, in one instance where the software recorded the trade twice and put me short by mistake.
The second reason that I have kept a journal is for learning and improving as a trader. I go back and review, trade by trade, what I was thinking and doing out of the ordinary in a given week that caused my results to be better or worse than average. And really, I don't need a lot of detail in the journal. For me, just having the trade and the time and any other comments, such as whether it was an active or quiet market, to help me re-live the day.
The third reason for keeping a journal is so I can find my way back, in the event I drift away from what I was doing when I was winning. It's like leaving a trail of pebbles so you can find your way home in case you get lost.