1) The name has the same sound as the word ‘Shobu (尚武)’ which means martial/Samurai spirit.
2) The shape of the very pointy leaves also reminded people of the Samurai sword so they wanted to place it at the entrance in order to get rid of the evil spirits coming into the house. These days people use Hana-Shobu to decorate their houses.
3) Shobu was originally believed to have a power to get rid of evil spirits. Its leaves have a strong scent and used medicinally. In the hope to have good health and a long life people started using the leaves in the bath.
4) Another Japanese word which has the same sound as ‘Shobu’ is ‘勝負’. It means ‘contest/match/game/win-defeat’. Obviously it is important for a Samurai family to win in a match so parents used Shobu for the Boy’s Day decoration by wishing the healthy growth of a boy who would become a great Samurai.