Why should i keep a journal?

...

To clean your ass when you don't have toilet paper

So people can laugh at you after you kill yourself

if you don't want to do it, then don't, i'm not your fucking mother

the only possible and correct answer

To practice giving coherence to your thoughts.

The correct answer
Edgelords

The only people who keep diaries are people not self aware that their thoughts aren't that special

If your thoughts were really that great merely mentioning them to other people would see your ideas spread

Your point doesn't make any sense. Keeping a diary doesn't mean you're telling other people your ideas. Maybe it's the opposite, you're keeping them for yourself. Or do you want to die without leaving your best?

to have an excuse to use your Moleskine™ Notebook