Kohei Horikoshi admits he created detailed lore on previous One For All users behind the scenes in My Hero Academia.

The vestiges were previous users of the quirk, and through their presence in the One For All, Deku unlocked their powers, magnified many times.

All the users of One For All, now known as the vestiges, have been extremely important, especially in Deku's fight against Shigaraki, and subsequently, All For One as well.