The OzTent RV-4 Fly is a convenient and useful accessory, but is NOT needed for waterproofing because the OzTent fly is already 100% waterproof without a fly. The important uses are to keep out condensation when temperatures drop, protect the top of your OzTent for long term, from the suns damaging UV rays, sap and bird droppings. Temperature control is also important during different seasons for comfort inside the tent. The RV Fly easily zips to the backside of the awning on your OzTent with a top of the line YKK zipper and is lifted off the tent with 2 poles which create an air exchange space, its so easy. The RV Fly has extended sides to protect the tents side windows which conveniently open from the inside. This allows you to have wonderful air flow no matter what the weather is like outside. The winged sides also create a great storage space for items you dont want inside the tent, like firewood, charcoal and the ice chest. With so many uses, it is well worth it to have the RV Fly.