#Leavanny is literally a seamstress/tailor, so I assume Leavanny silk is used to make some clothing. Even though #Tarountula looks like a ball of yarn, can eject that huge ball of silk, and has a #Shiny that looks like the Destiny Knot, there's nothing about its silk being used. And apparently #Spinarak silk is sometimes used for fishing nets (nothing about clothing, though).

Edit: Apparently #Ariados silk is used for weaving.