I've always kept a diary.It's a good way to recall your teenage thoughts for instance,to recall if you were really that unhappy as you thought.
It's great when raising children to keep dates of illnesses like measles,chicken pox and especially in girls,if and when they had german measles.This is important to know later when they're at child-bearing age.
I've always wrote the dates that were important in our lives,like birthdays,first day of school,what the children liked and didn't like in food. Also it's good to keep track of allergies and accidents they might have had.
Besides there's always favorite toys and animals that were once part of the household and have since gone. These things are a delight to read in later years especially if you are a writer.
And if you're sad and troubled,writing your thoughts makes your heart lighter.