As others have mentioned, it's a great way to keep oneself accountable. It's like a reminder - hey, am I doing writing stuff like I aim to, or am I neglecting my stories, submissions, etc. It can serve as a motivator, like, ah crap I haven't done shit for the past 3 weeks, I should write another short and enter a contest or something.
What I've really enjoyed has been tracking the numbers, specifically for novel writing and for time spent querying/submitting. This can be motivating as well. For example, here are the stats for my most recent novel:
136 hours writing
94.5 hours editing
230.5 hours total
Sometimes writing a new novel seems an impossible task, but with numbers you are reminded that it's absolutely doable, even if you're only writing on days off and weekends, as I do. 230 hours? Yeah, I can set aside that much time over 6-8 months, easy. And then you can get into the fun stats like average words per day, words per session, all kinds of stuff.
Keeping a progress journal helps keep me focused and is a constant reminder that I gotta keep going with the writing activities.