The Auto button, added to the Crop tool in Lightroom CC / 6, allows customers to apply an adjustment similar to Level in the Upright function in the Lens Correction panel. As many users are accustomed to adjusting angle from within the Crop panel, having access to an auto-straightening option eliminates the need for users to go to a different panel to try an auto option before final tweaking. Not all users dive into the Lens Correction panel but most use the Crop panel.
Note: the same action can be accomplished by [Shift]+[Double-Click] on the Angle slider label.