While you may not need them all the time, revisions can be very useful if you need to recover an older version of a post of page, or just to see what you wrote in a previous save.
If you don't see the revisions box on your edit screen, turn it on from the screen options. By default, it would be under the content editor, but it can be moved anywhere you want it. Regardless of if you have the revisions box displayed on your screen any changes you have made to your content will be seen here. WordPress makes a new revision every time you save a draft publish or update published content as well as periodical autosaves.
View and Restore Revisions
Clicking on a revision will take you to the revision comparison page or click browse next to revisions in your publish box. Use the previous and next buttons to scroll through your revisions or drag your slider to jump through the timeline. In the main window you can see the changes that have been made to your content.
The items highlighted in red are the before and the items highlighted in green are the after. Clicking "restore this revision" will restore the content highlighted in green.
By default, this only compares consecutive revisions. If you wish to compare your current save with a save that is further back, click the box next to ‘compare any two revisions’, this will split your slider allowing you to compare any two revisions on the timeline. In order to restore the older version you will need to scroll the slider on the right to the position of the one on the left. And click ‘restore this revision’. This will take you back to the edit page where you can update and publish your content.