Firstly, journaling is not like keeping a diary. Writing in a diary is usually to record events and main feelings of the day and serves as an aid-memoir, while journaling is usually to achieve a goal, commonly involves answering questions designed to expand your introspection further or help with reflection, and is not necessarily performed daily - although it can be.
It can help you organise your thoughts and ideas in a coherent way.
Writing consistently can reveal patterns of behaviour or thought which can then be addressed with more focus.
You can track growth and development through journaling and increase self-awareness.
Writing between therapy sessions can allow something significant to come to the surface which may have taken a longer time to come up through just talking, before addressing it in therapy.
You can unlock and examine various feelings held about certain events and people.
If you choose to support your therapy with a journaling practice then take care with selecting the right notebook for you - it has to be appealing to encourage you to write. My primary thought journal is a lined A4 spiral-bound notebook, because I find anything smaller to be rather limiting when I am writing stream of consciousness style and emptying my mind.
What about using a computer instead? While individual differences vary (and these can be considered in therapy of course!), studies have shown that writing by hand activates more areas of the brain, which is beneficial in itself, but choosing to write on paper can allow your thoughts to come out quicker - even if you do not write as fast as you type - which is why it's generally recommended.
If you would like to work with me as a therapist, mentor and coach and would like to incorporate journaling too then let me know and get in touch using the contact form.