This is more like a note to self, than for the readers.
Now, like a obedient class 7 kid, I will write the answer.
I should write my personal diary because :
1. It gives me a chance to reflect on the day's happenings & my reaction to them. It should help me, should I choose, improve my Sambhav & help me become a better human being with control on my emotions & my actions & reactions.
2. I write about the positive things mostly which helps me keep a positive view towards life.
3. Writing my diary gives me a chance to read it which gives me a third person's perspective on my life & me, which helps me evaluate myself & correct my faults . At the least, it can help me to not repeat my mistakes.
4. At t=t, it will become a memory should I choose to recollect & read it.