Catharsis is emotional release. This is one of the primary motivations most people have for keeping a diary.
You can use your journal to purge yourself of emotion any time you wish. For example, you may be so angry at someone that you want to scream and yell. However, you know that this would destroy the relationship. So you write a page or 20 pages about your anger and the reasons for it. Somewhere in your emotional outpouring you'll sense a shift. You won’t be quite as enraged any more.
Purging yourself in your journal can be very healthy.