I've kept a diary off and on most of my adult life. (I also kept diaries as a teen but shortly before I left for college I lit a bonfire and burned them in a grand melodramatic style.) There are a number of reasons why people keep diaries, besides needing something to read on the train, but I've never been sure where I fall on that spectrum. A large part is blowing off steam, of course. The diary will never judge you, no matter how much you whine about traffic and stupid people and waiting at the doctor's office. There's also, on a more practical note for a writer, the art of description -- the opportunity for a word picture of the person sitting opposite you in the doctor's waiting room. When I lived in NYC I kept a "Street Diary," which was just descriptions of people and/or scenes I had witnessed on the streets and subways, scribbled down irregularly. This was great practice for writing and I recommend it to anyone but it's hard to keep it up in suburbs, where there is less public space.
The constantly true thing about any diary is that the important stuff never gets written down or gets written down too late. Big emotional issues are too large to grapple with on daily basis most of the time. Sometime things that become important start out very small and by the time they are important it's too late to describe them accurately. But I keep going, hoping I'm capturing at least part of myself at this particular stage of life.
I think writers fall into two categories -- internal and external. Some writers burn entirely off their own emotions and all their creations stem from their own central dramas. (Writers who live highly dramatic lives, like F. Scott Fitzgerald or Patricia Highsmith, often fall into this first category.) Others don't look inside themselves much but feed off observing and analyzing people. I fall mostly into the second category. I'm fascinated by people. I think I'm going to try and re-orient my diary into less blowing off steam and more looking outward and observing daily life.