That's right. If a slide deflates on the main doors it can be used as a rag slide with two people pulling on and holding it at the bottom. That doesn't quite work at the over wings unless the slide is attached to the wing. So on the overwing dual lane slide there is a clip that can be attached to the bracket which enables someone at the bottom to use it as a rag slide.
It's also the attachment point for the parachute for that wing (obviously there is one for each side). (it is at the C of G).
In the event of a sudden, alarming descent which the pilot decides is never going to become controllable, he operates an alarm to tell the CC to open the emergency window, and take the parachute pack out on to the wing and clip it on to the bracket you can see.
The parachute pack is stowed under the exit seat for that purpose.
The same procedure is followed for each wing. As soon as the parachute is secure the CC comes back into the cabin bringing the release cord. The releases for both sides are pulled simultaneously to prevent asymetric uplift.
The aircraft then descends in a stately manner to the surface, while passengers don their lifejackets in the approved fashion if over water, record the video for YouTube, text their families and the press with details of their terrifying ordeal, get their hand luggage ready for leaving the aircraft, go for an emergency pee,and so on.
A320s I've operated use them to attach guide ropes to. These are stowed in the small overhead locker above the overwing exits and attach to a bracket on the inside of the exit, they are then attached to the brackets on the wing. From memory, this was just for ditching.