Diary Writing is a good skill which everyone should have. It's not just about writing a diary, your personality reflects from what you write. That's why many renowned authors & writers are known for their specific style & forte of writing, some writes fiction, drama, thriller, etc. You write what you believe in. Diary writing helps in self-assessment. You know what you are good & bad at. You make changes in your life as required.
Daily diary entry brushes up your writing skills. You note down goals & steps, which help in achieving it easily because every day whenever you open up the same page, you know what to do. It keeps you away from stress. It's well said, "Diary is your best friend" because it knows you better than anyone else. You write your own expressions, opinions & views which releases stress level also. Your mind becomes free & always ready to work happily.
Writing habit is a good for students specially. More you write, more you'll learn, more grammatically perfect, better writing skills. School students should do this writing exercise every day. One page about how was the day, the best thing happened in the day & the worst thing happened as well, which keeps them away from tension & frustrations; they'll be more physically fit. Students who are facing family tensions should make a routine to express their feelings a page or two, so they can feel that yes, there is a friend with whom they can share all pain & sufferings also.
In first world countries, mostly people are involved in writing Journals which keeps a record of the event happened in present. After so many years when you'll come back & read your written stuff, you'll feel great where you were & where you reached.
There is no need to write long paragraphs as if you are writing a story. Write short, but it should reflect real you. It tracks your daily record & you don't require any special skill to write it. Write free & real.