You can have all the recovery straps, winches, and more, but if you don't have a secure way to attach them to your Jeep, you'll be left up the creek without a paddle! D-rings, or shackles as they're known allow you to safely attach recovery gear to you or a friends Jeep to get back on the trail when you end up in a stuck situation.
D-ring accessories, like isolators are popular add-on products as these polyurethane clips slide against the D-Ring loop to prevent banging against a bumper or other recovery products.