ADHD can make it difficult for people to navigate through their thoughts and emotions. Journaling can help slow things down for them and allow them to focus better on these things.
Journaling causes your mind to slow down, which can allow you to catch up with it and actually understand what you are going through, what you are feeling, and what is on your mind. It also allows you to better focus on a single item, whether you are writing about an event that happened, a feeling or emotion, or a thought you are trying to explore.
It is important, though, to understand that journaling may take time to get better at. For some it may come more naturally, for others, especially those who are not used to writing things down, it could be a little bit more difficult to start the practice. The key is to keep at it and to not get frustrated or impatient if it doesn't work out right away.