there are many benefits of writing in a journal or a diary... i have been writing since i was really young... and its a good way for me to reflect and express how i feel...
it definitely improves your writing skills because you free write and you are able to express your thoughts on papers... many of my professors have said that writing in journals/ diaries are important in improving your skills
plus i like it cause i can always look back on my feelings or events that have happened in my past... or just to rekindle some of my past ideas and things that used to motivate me...
you should really consider writing everyday or a few times a week because there is a very rewarding outcome... =)