I do the same thing a lot, I find it's often a case that I realize i'm just being a dick or have some sort of emotional reaction, some I realize I'm just adding nothing.

I do this far more often with e-mail, sometimes i'll be e-mailing my friend about something, and then all of a sudden go on a big tangent about a problem i'm having or facing and by the time I'm finished the e-mail it's a novel, but I fleshed out my problems and it helped me immensely. I just delete the e-mail at the end / send the one liner response I had in the first place.

Writing to someone else comes from a different perspective then just journaling for yourself.