I don't keep a journal, save for Hallowe'en, but a lot of what I do is logged somewhere, much of it on this blog. If I exchange what I consider significant mail or email with somebody, somewhere these is a copy of it.
This activity has pros and cons, but one of the good things is that, if I have ever been in a discussion with somebody via mail or email and there is some question about who said what, when? If I know an approximate date, I can find the discourse and review it.
Sometimes my memory isn't crystal clear, sometimes it's outright wrong, but having it written down helps. Doesn't make it any truer if it is on paper or a file I can access electronically, but it does show what actually got said, by whom.
Writers are taught not to throw their work away. Someday, they can mine it for material, or leave it to some university after they shuffle off, and provide some desperate grad student a shot at a thesis or maybe a biography of some minor celebrity.
And it comes in handy when you are arguing about that time the doorknob broke and you can point out that you said so ...