When you gopes to shrine you will find a straw rope with white paper “Shide” is hanged.

This is a seal to separate human world and God world. If the area is surrounded with Shimenawa, the inside will be separated as same as God world.
Sumo champion is only one exception to weare Shimenawa. For the first day of the year, people decorate their entrance and car with Shimenawa as Shintoism reason.
The material for Shimenawa is as mentioned, but it has to use after harvest rice straw.
Perhaps, it is related with rice is gift of god and nature since the ancient time.
personally