You often start with documents/ articles and cannot complete the task at hand. You may not want to publish any unfinished work.
In such cases, drafts are useful when you want to save any work-in-progress items. For every item that you compose, at the bottom you will see this button .
Click on the "Save as Draft" button, and access all your drafts by going to the Drafts tab available on the homepage (shown below)
By default, when you save any item as a draft, the access rights for the item are set to be visible only to you. Unless, if you have specified users/teams to be able to view or edit the item.