Is a dream journal all that necessary? I find that atleast when I remember my dreams that I'm doing something right. I mean I hash over my dreams and their relevance. I've also lately had dreams that have had quick turn-around upon relevance. So really what's the worth in keeping a journal?
It's not completely necessary, no.
But it is helpful.
Keeping a dream journal helps with dream recall by telling your subconscious mind that you want to start remembering these events.
A dream journal can also help with assisting you to have more Lucid Dreams... the more dreams you write down, the more "dream events" you're bringing into your conscious mind. The more "dream events" you can do that with, the greater the chance that you'll recognize them when they're actually happening within a dream you're having... which then allows you to say "Oh hey, I read this in my journal once... I think I'm dreaming now!".