Probably, shirasaya was first introduced in families of the upper-class samurai, mainly daimyo (Japanese feudal lord) who had a collection of swords, to preserve the swords with care, which were regarded as an essential tool of samurai.
大名等、蔵刀が多い上級の武家では、武士の表道具と言われる刀を大切に保存するために、白鞘を用い始めたのであろう。