I used to write in my diary all the time. It would be about whatever: stories, ideas of stories, day-to-day happenings, movies I watched, etc. Sometimes, I would write in it every single day, every couple of hours. Other times, I would crack it open every few months, wracking my brain to scribble down even a few words.
Whatever the reason for writing in it and no matter how often you did write in it, a diary can be useful in not only your mental capabilities but also your emotional ones.
According to a Nov. 2018 Psychologies article, a diary - and writing in it - can act as a "therapist and a dear friend." Sometimes, we can be stuck in our own feelings, and it may feel like either you don't need to let them out or that there is no one to listen. Either way, a diary will always be there for you to vent.
From personal experience, I find that when I am feeling angry, lonely, or sad, I use my diary the most. Having the outlet to scratch your thoughts is immensely therapeutic. Moreover, no one will probably ever read your diary, so it is like an extension of your mind.
Also, a diary doesn't need to be expensive: you don't need any fancy materials, and in fact, just sheets of paper stapled together can act as your diary. Diaries are accessible and can mold to your own needs whether they are big or small.
According to July 2016 HelloGiggles article, you can write in your diary to "preserve your brightest memories." Even if you never write in it again, who knows that several years later, you will find it in the depths of your moving cardboard boxes; you sit down and flip it open, and suddenly, you are transported into a world that you can have the pleasure of revisiting. In a sense, a diary is a place that you can reflect in freely.
Writing in a diary can be stressful as it can be a harbinger of unnecessary pressure. For that, I will say that I don't write in my diary every day. Heck, there are times when I don't even open it up for weeks on end. However, that is okay. The best thing about a diary is that even if you never touch it for years on end, you can always find it waiting for you to write in it.
If you don't have a diary, trust me: it is the best. I can write about anything and everything and nothing at all in it; there are no limitations to writing in a diary. When you do write in it though, it is therapeutic, fun, and nostalgic all at once.