Duct tape is small and light so it's easy to keep in the trunk and can save you in a bind for situations like keeping your muffler attached temporarily. Honestly, once you start carrying around duct tape you'll be surprised at all the applications it has. Did someone knock off one of your side mirrors? Duct tape will work as a temporary fix until you can get it repaired. Small tear in your upholstery? Seal it up with duct tape ASAP to keep it from growing larger.