Dreams offer valuable insights into your motivations. Keeping a dream diary allows you to analyse them at a deeper level.

When you keep a dream diary you creating a conversation with your subconscious whose primary language is dream imagery. The more you use your diary the more you will learn about yourself as dreams contain the most intimate details about who you are and how you can fulfil your own life. By keeping a dream diary, you'll discover many things about your intentions, as well as the intentions of those around you. You will also gain insight into your dormant skills or abilities.

It is up to you how you record your dreams, but there are somethings you may wish to include, that will help when interpreting your dreams.
It is recomended that your record the date of the dream, time of waking, main theme, significant characters and objects, actions, atmosphere, special comments, and real life connections.

I kept a dream diary a bit back for amongst other things the above reasons;the other day I went in a building and had a huge deja vu...which I realised was from one of the dreams I'd written in my diary...so I was able to go and check I was right...so for this I was dead happy if nothing else hehehehehe!
Funnily enough I was also keeping a meditation diary too and some of my dreams and meditiaitions were crossing over...and this deja vu was of one of those which crossed over...which esp with the timing of it is very very interesting...not sure to explain in words but I know what I mean!!!