Shimenawa, the ropes usually made of rice-straw though increasingly of plastic, are found in many places in Japan though typically at shrines. They can be found wrapped around sacred rocks, sacred trees, strung across torii, shrine entrance gates, and across shrine structures. they come in a huge diversity of styles and sizes up to the 5 ton giant at Izumo Taisha.
There are many theories as to the origin of shimenawa, many connecting it to the use of rope to mark possession of things or space, and I have also read that it originates in the rope tied around the campsite of central Asian nomads, but the simple truth is nobody knows for sure.
When it comes to the mythological origin of the shimenawa however we are given two different origins, one connected to Amaterasu and the other to Susano. Not surprisingly the myth connected to Amaterasu is by far the dominant.
According to the Iwato myth, wherein Amaterasu hides herself away in a cave and plunges the world into darkness, she is finally tempted out by the dance of Uzume, and after Tajikarao pulls her out another kami stretches a rope across the cave entrance to stop Amaterasu from going back in. I realize that myths do not have to make sense, but this story makes no sense at all because if a shimenawa is supposed to stop a kami from entering a space, then why are they used to mark space that is inhabited by kami?
In Okayama there is a story that tells how Susano instructed the local people how to make a chinowa, a hoop woven out of reeds or sometimes rice straw that by passing through purifies the person. Further north in Tottori a similar story tells how in return for a kindness Susano teaches a local man how to string a rope along the street to purify it and keep out disease. Shimenawa mark sacred space, and in Japan the sacred is equated with purity, so these stories make much more sense.
So why is the Amaterasu version the most common? Since the beginning of recorded history the Yamato have been denigrating Susano and elevating Amaterasu. Much of contemporary "shinto" is still tied to Imperial Shinto and the State Shinto of the late 19th Century. These forms of shinto placed the imperial family and Amaterasu at the apex and this is still put forward today. The Yamato were relative latecomers, even their own myths say that Susano and his descendants ruled over "Japan" before they did, so it makes sense that some of Japanese culture must originate from Izumo traditions.