A draft post is only editable through the administration panel. Only the post author and those that have editor or administrator user roles can edit the draft.
The advantage of having a post in draft form is the ability to save it for later editing prior to its publishing. By default when a post is saved it is saved in draft form. It does not get published until the author or editor (i.e. whoever has appropriate permissions) actually hits the publish button or schedules it for a future publish.
Within the WordPress editor, under the Publish meta box, use the “Save Draft” button instead of the “Publish” button to save your current post in draft form without publishing it.