It will help your brain stay organized.
There are a million kinds of journaling out there, not just baring your soul in a bright pink diary. Many forms of journaling more closely parallel list making. The latest rage in list-style journaling are Bullet Journals. They take a few tries to grasp, but then become a very efficient mix of record keeping and to do list. As your wedding gets closer and closer and more items need your attention, you'll be grateful for the organization.
You need an outlet to stay sane.
Weddings can be highly emotional and stressful (even if it is all positive). If you find yourself needing an outlet, a journal might be the perfect tool to helping you stay sane (not to mention that some of your thoughts might just evolve into your wedding vows). Just remember that if you are going to tell-all, to keep it in a safe space. Leaving it out on the counter for your future mother-in-law to come across would probably not help your stress levels.
A journal can be a mirror.
Whether your journal entries are pages-long or just a few sentences, you can use these passages to reflect later. Sometimes our own words can be the best mirror to our behaviors and patterns. For example, if you notice that your entries become more negative whenever you don't have enough alone time, make some small changes to give yourself that space.
Your journal can be a keepsake just for you.
Being engaged is such an exciting and romantic chapter. Keep a journal to look back on this amazing period as you started your new life together. You can even add scrapbook components such as photos and concert tickets.
Or a love letter to your new spouse.
Another form of journaling is letter writing. Each entry could be a letter to your future spouse. You could surprise him with it on the morning of your wedding, or let him in on the fun and do it together. Each of you could enclose letters in envelopes to read on future anniversaries or specific life moments ahead.