I write to think through my life, one conundrum or experience at a time.
First, collect bits. Evidence or insights or confounding experience.
And then see where these go. Or even what sticks and what slips away.
THE SHORT ANSWER
You never know what we'll churn up in cleaning a stall
Jnana's Red Barn 
They say that writing helps to sort out your life, and I think it does. Gives you some valuable perspective, almost as good as talking to someone…
Yes! That points us to the importance of keeping a journal, too, something I posted about several years ago.